diff --git a/optuna_dashboard/preferential/samplers/gp.py b/optuna_dashboard/preferential/samplers/gp.py
index beb5b7c23..3cd980614 100644
--- a/optuna_dashboard/preferential/samplers/gp.py
+++ b/optuna_dashboard/preferential/samplers/gp.py
@@ -50,15 +50,15 @@ def _orthants_MVN_Gibbs_sampling(cov_inv: Tensor, cycles: int, initial_sample: T
 
 
 def _one_side_trunc_norm_sampling(lower: Tensor) -> Tensor:
-    if lower > 4.0:
-        r = torch.clamp_min(torch.rand(torch.Size(()), dtype=torch.float64), min=1e-300)
-        return (lower * lower - 2 * r.log()).sqrt()
-    else:
-        SQRT2 = math.sqrt(2)
-        r = torch.rand(torch.Size(()), dtype=torch.float64) * torch.erfc(lower / SQRT2)
-        while 1 - r == 1:
-            r = torch.rand(torch.Size(()), dtype=torch.float64) * torch.erfc(lower / SQRT2)
-        return torch.erfinv(1 - r) * SQRT2
+    r = torch.rand(torch.Size(()), dtype=torch.float64)
+    ret = -torch.special.ndtri(torch.exp(torch.special.log_ndtr(-lower) + r.log()))
+
+    # If sampled random number is very small, `ret` becomes inf.
+    while torch.isinf(ret):
+        r = torch.rand(torch.Size(()), dtype=torch.float64)
+        ret = -torch.special.ndtri(torch.exp(torch.special.log_ndtr(-lower) + r.log()))
+
+    return ret
 
 
 _orthants_MVN_Gibbs_sampling_jit = torch.jit.script(_orthants_MVN_Gibbs_sampling)
diff --git a/python_tests/preferential/samplers/test_gp.py b/python_tests/preferential/samplers/test_gp.py
new file mode 100644
index 000000000..136959a79
--- /dev/null
+++ b/python_tests/preferential/samplers/test_gp.py
@@ -0,0 +1,29 @@
+import sys
+from unittest.mock import patch
+
+import numpy as np
+import pytest
+
+
+if sys.version_info >= (3, 8):
+    from optuna_dashboard.preferential.samplers.gp import _one_side_trunc_norm_sampling
+    import torch
+else:
+    pytest.skip("BoTorch dropped Python3.7 support", allow_module_level=True)
+
+
+def test_one_side_trunc_norm_sampling() -> None:
+    for lower in np.linspace(-10, 10, 100):
+        assert _one_side_trunc_norm_sampling(torch.tensor([lower], dtype=torch.float64)) >= lower
+
+    with patch.object(torch, "rand", return_value=torch.tensor([0.4], dtype=torch.float64)):
+        sampled_value = _one_side_trunc_norm_sampling(torch.tensor([0.1], dtype=torch.float64))
+        assert np.allclose(sampled_value.numpy(), 0.899967154837563)
+
+    with patch.object(torch, "rand", return_value=torch.tensor([0.8], dtype=torch.float64)):
+        sampled_value = _one_side_trunc_norm_sampling(torch.tensor([-2.3], dtype=torch.float64))
+        assert np.allclose(sampled_value.numpy(), -0.8113606739551955)
+
+    with patch.object(torch, "rand", return_value=torch.tensor([0.1], dtype=torch.float64)):
+        sampled_value = _one_side_trunc_norm_sampling(torch.tensor([5], dtype=torch.float64))
+        assert np.allclose(sampled_value.numpy(), 5.426934003050024)