diff --git a/optuna/artifacts/_backoff.py b/optuna/artifacts/_backoff.py
index f8b6ce5f70..823a9c62b2 100644
--- a/optuna/artifacts/_backoff.py
+++ b/optuna/artifacts/_backoff.py
@@ -13,7 +13,7 @@
 if TYPE_CHECKING:
     from typing import BinaryIO
 
-    from ._protocol import ArtifactStore
+    from optuna.artifacts._protocol import ArtifactStore
 
 
 class Backoff:
@@ -112,6 +112,6 @@ def remove(self, artifact_id: str) -> None:
 if TYPE_CHECKING:
     # A mypy-runtime assertion to ensure that the Backoff middleware implements
     # all abstract methods in ArtifactStore.
-    from . import FileSystemArtifactStore
+    from optuna.artifacts import FileSystemArtifactStore
 
     _: ArtifactStore = Backoff(FileSystemArtifactStore("."))
diff --git a/optuna/artifacts/_boto3.py b/optuna/artifacts/_boto3.py
index 62366131a4..ed9cd7f4ee 100644
--- a/optuna/artifacts/_boto3.py
+++ b/optuna/artifacts/_boto3.py
@@ -101,6 +101,6 @@ def _is_not_found_error(e: ClientError) -> bool:
 if TYPE_CHECKING:
     # A mypy-runtime assertion to ensure that Boto3ArtifactStore implements all abstract methods
     # in ArtifactStore.
-    from ._protocol import ArtifactStore
+    from optuna.artifacts._protocol import ArtifactStore
 
     _: ArtifactStore = Boto3ArtifactStore("")
diff --git a/optuna/artifacts/_filesystem.py b/optuna/artifacts/_filesystem.py
index dcdfcbb740..32318996bd 100644
--- a/optuna/artifacts/_filesystem.py
+++ b/optuna/artifacts/_filesystem.py
@@ -75,6 +75,6 @@ def remove(self, artifact_id: str) -> None:
 if TYPE_CHECKING:
     # A mypy-runtime assertion to ensure that LocalArtifactBackend
     # implements all abstract methods in ArtifactBackendProtocol.
-    from ._protocol import ArtifactStore
+    from optuna.artifacts._protocol import ArtifactStore
 
     _: ArtifactStore = FileSystemArtifactStore("")
diff --git a/optuna/artifacts/_gcs.py b/optuna/artifacts/_gcs.py
index ec3826d623..65aa97a951 100644
--- a/optuna/artifacts/_gcs.py
+++ b/optuna/artifacts/_gcs.py
@@ -89,6 +89,6 @@ def remove(self, artifact_id: str) -> None:
 if TYPE_CHECKING:
     # A mypy-runtime assertion to ensure that GCS3ArtifactStore implements all abstract methods
     # in ArtifactStore.
-    from ._protocol import ArtifactStore
+    from optuna.artifacts._protocol import ArtifactStore
 
     _: ArtifactStore = GCSArtifactStore("")
diff --git a/optuna/samplers/_gp/__init__.py b/optuna/samplers/_gp/__init__.py
index 9f55ab1f62..ff09f1235a 100644
--- a/optuna/samplers/_gp/__init__.py
+++ b/optuna/samplers/_gp/__init__.py
@@ -1,4 +1,4 @@
-from .sampler import GPSampler
+from optuna.samplers._gp.sampler import GPSampler
 
 
 __all__ = ["GPSampler"]