diff --git a/docs/schema/requirement.md b/docs/schema/requirement.md
index 0717efe..930d2e0 100644
--- a/docs/schema/requirement.md
+++ b/docs/schema/requirement.md
@@ -44,6 +44,12 @@
 | `REF013` | `sample-data-filename-presence`       | `ERROR`  | `SampleData.filename` exists.                                             |
 | `REF014` | `sample-data-info-filename-presence`  | `ERROR`  | `SampleData.info_filename` exists if it is not `None`.                    |
 | `REF015` | `lidarseg-filename-presence`          | `ERROR`  | `LidarSeg.filename` exists if `lidarseg.json` exists.                     |
+| `REF016` | `sample-next-to-another`              | `ERROR`  | `Sample.next` refers to another one unless it is empty.                   |
+| `REF017` | `sample-prev-to-another`              | `ERROR`  | `Sample.prev` refers to another one unless it is empty.                   |
+| `REF018` | `sample-annotation-next-to-another`   | `ERROR`  | `SampleAnnotation.next` refers to another one unless it is empty.         |
+| `REF019` | `sample-annotation-prev-to-another`   | `ERROR`  | `SampleAnnotation.prev` refers to another one unless it is empty.         |
+| `REF020` | `sample-data-next-to-another`         | `ERROR`  | `SampleData.next` refers to another one unless it is empty.               |
+| `REF021` | `sample-data-prev-to-another`         | `ERROR`  | `SampleData.prev` refers to another one unless it is empty.               |
 
 ## Format (`FMT`)
 
diff --git a/t4_devkit/sanity/reference/__init__.py b/t4_devkit/sanity/reference/__init__.py
index 4aefae1..d61ed05 100644
--- a/t4_devkit/sanity/reference/__init__.py
+++ b/t4_devkit/sanity/reference/__init__.py
@@ -15,3 +15,9 @@
 from .ref013 import *  # noqa
 from .ref014 import *  # noqa
 from .ref015 import *  # noqa
+from .ref016 import *  # noqa
+from .ref017 import *  # noqa
+from .ref018 import *  # noqa
+from .ref019 import *  # noqa
+from .ref020 import *  # noqa
+from .ref021 import *  # noqa
diff --git a/t4_devkit/sanity/reference/ref016.py b/t4_devkit/sanity/reference/ref016.py
new file mode 100644
index 0000000..3f02393
--- /dev/null
+++ b/t4_devkit/sanity/reference/ref016.py
@@ -0,0 +1,20 @@
+from __future__ import annotations
+
+from t4_devkit.schema import SchemaName
+
+from ..checker import RuleID, RuleName, Severity
+from ..registry import CHECKERS
+from .base import RecordReferenceChecker
+
+
+@CHECKERS.register()
+class REF016(RecordReferenceChecker):
+    """A checker of REF016."""
+
+    id = RuleID("REF016")
+    name = RuleName("sample-next-to-another")
+    severity = Severity.ERROR
+    description = "'Sample.next' refers to another one unless it is empty."
+    source = SchemaName.SAMPLE
+    target = SchemaName.SAMPLE
+    reference = "next"
diff --git a/t4_devkit/sanity/reference/ref017.py b/t4_devkit/sanity/reference/ref017.py
new file mode 100644
index 0000000..ffaaba9
--- /dev/null
+++ b/t4_devkit/sanity/reference/ref017.py
@@ -0,0 +1,20 @@
+from __future__ import annotations
+
+from t4_devkit.schema import SchemaName
+
+from ..checker import RuleID, RuleName, Severity
+from ..registry import CHECKERS
+from .base import RecordReferenceChecker
+
+
+@CHECKERS.register()
+class REF017(RecordReferenceChecker):
+    """A checker of REF017."""
+
+    id = RuleID("REF017")
+    name = RuleName("sample-prev-to-another")
+    severity = Severity.ERROR
+    description = "'Sample.prev' refers to another one unless it is empty."
+    source = SchemaName.SAMPLE
+    target = SchemaName.SAMPLE
+    reference = "prev"
diff --git a/t4_devkit/sanity/reference/ref018.py b/t4_devkit/sanity/reference/ref018.py
new file mode 100644
index 0000000..d894310
--- /dev/null
+++ b/t4_devkit/sanity/reference/ref018.py
@@ -0,0 +1,20 @@
+from __future__ import annotations
+
+from t4_devkit.schema import SchemaName
+
+from ..checker import RuleID, RuleName, Severity
+from ..registry import CHECKERS
+from .base import RecordReferenceChecker
+
+
+@CHECKERS.register()
+class REF018(RecordReferenceChecker):
+    """A checker of REF018."""
+
+    id = RuleID("REF018")
+    name = RuleName("sample-annotation-next-to-another")
+    severity = Severity.ERROR
+    description = "'SampleAnnotation.next' refers to another one unless it is empty."
+    source = SchemaName.SAMPLE_ANNOTATION
+    target = SchemaName.SAMPLE_ANNOTATION
+    reference = "next"
diff --git a/t4_devkit/sanity/reference/ref019.py b/t4_devkit/sanity/reference/ref019.py
new file mode 100644
index 0000000..02dc719
--- /dev/null
+++ b/t4_devkit/sanity/reference/ref019.py
@@ -0,0 +1,20 @@
+from __future__ import annotations
+
+from t4_devkit.schema import SchemaName
+
+from ..checker import RuleID, RuleName, Severity
+from ..registry import CHECKERS
+from .base import RecordReferenceChecker
+
+
+@CHECKERS.register()
+class REF019(RecordReferenceChecker):
+    """A checker of REF019."""
+
+    id = RuleID("REF019")
+    name = RuleName("sample-annotation-prev-to-another")
+    severity = Severity.ERROR
+    description = "'SampleAnnotation.prev' refers to another one unless it is empty."
+    source = SchemaName.SAMPLE_ANNOTATION
+    target = SchemaName.SAMPLE_ANNOTATION
+    reference = "prev"
diff --git a/t4_devkit/sanity/reference/ref020.py b/t4_devkit/sanity/reference/ref020.py
new file mode 100644
index 0000000..5afa2c0
--- /dev/null
+++ b/t4_devkit/sanity/reference/ref020.py
@@ -0,0 +1,20 @@
+from __future__ import annotations
+
+from t4_devkit.schema import SchemaName
+
+from ..checker import RuleID, RuleName, Severity
+from ..registry import CHECKERS
+from .base import RecordReferenceChecker
+
+
+@CHECKERS.register()
+class REF020(RecordReferenceChecker):
+    """A checker of REF020."""
+
+    id = RuleID("REF020")
+    name = RuleName("sample-data-next-to-another")
+    severity = Severity.ERROR
+    description = "'SampleData.next' refers to another one unless it is empty."
+    source = SchemaName.SAMPLE_DATA
+    target = SchemaName.SAMPLE_DATA
+    reference = "next"
diff --git a/t4_devkit/sanity/reference/ref021.py b/t4_devkit/sanity/reference/ref021.py
new file mode 100644
index 0000000..b80aec0
--- /dev/null
+++ b/t4_devkit/sanity/reference/ref021.py
@@ -0,0 +1,20 @@
+from __future__ import annotations
+
+from t4_devkit.schema import SchemaName
+
+from ..checker import RuleID, RuleName, Severity
+from ..registry import CHECKERS
+from .base import RecordReferenceChecker
+
+
+@CHECKERS.register()
+class REF021(RecordReferenceChecker):
+    """A checker of REF021."""
+
+    id = RuleID("REF021")
+    name = RuleName("sample-data-prev-to-another")
+    severity = Severity.ERROR
+    description = "'SampleData.prev' refers to another one unless it is empty."
+    source = SchemaName.SAMPLE_DATA
+    target = SchemaName.SAMPLE_DATA
+    reference = "prev"