diff --git a/Optuna/Study/Study.cs b/Optuna/Study/Study.cs
index c65dc5e4..4db4d7bb 100644
--- a/Optuna/Study/Study.cs
+++ b/Optuna/Study/Study.cs
@@ -105,7 +105,7 @@ public static StudySummary[] GetAllStudySummaries(IOptunaStorage storage)
                 IEnumerable<Trial.Trial> completeTrials = allTrials.Where(trial => trial.State == TrialState.COMPLETE);
 
                 StudyDirection direction = StudyDirection.NotSet;
-                StudyDirection[] directions = Array.Empty<StudyDirection>();
+                StudyDirection[] directions = null;
                 Trial.Trial bestTrial = null;
                 if (studies[i].Directions.Length == 1)
                 {
diff --git a/Optuna/Study/StudySummary.cs b/Optuna/Study/StudySummary.cs
index a927d919..cbe46b28 100644
--- a/Optuna/Study/StudySummary.cs
+++ b/Optuna/Study/StudySummary.cs
@@ -66,10 +66,6 @@ public StudyDirection[] Directions
         {
             get
             {
-                if (_directions.Length == 1)
-                {
-                    throw new InvalidOperationException("The study has only one direction.");
-                }
                 return _directions;
             }
         }
diff --git a/OptunaTests/Storage/StorageSameBevaviorTests.cs b/OptunaTests/Storage/StorageSameBevaviorTests.cs
index dbd79631..1b60523e 100644
--- a/OptunaTests/Storage/StorageSameBevaviorTests.cs
+++ b/OptunaTests/Storage/StorageSameBevaviorTests.cs
@@ -7,7 +7,7 @@ namespace Optuna.Storage.Tests
 {
     public class StorageSameBehaviorTests
     {
-        [Theory()]
+        [Theory]
         [InlineData(@"TestFile/sqlite.db", "sqlite")]
         [InlineData(@"TestFile/journal.log", "log")]
         public void StorageLoadTest(string path, string type)
diff --git a/OptunaTests/Study/StudySummaryTests.cs b/OptunaTests/Study/StudySummaryTests.cs
new file mode 100644
index 00000000..dec7e9a2
--- /dev/null
+++ b/OptunaTests/Study/StudySummaryTests.cs
@@ -0,0 +1,29 @@
+using System;
+
+using Optuna.Storage;
+using Optuna.Storage.Journal;
+using Optuna.Storage.RDB;
+
+using Xunit;
+
+namespace Optuna.Study.Tests
+{
+    public class StudySummaryTests
+    {
+        [Theory]
+        [InlineData(@"TestFile/sqlite.db", "sqlite")]
+        [InlineData(@"TestFile/journal.log", "log")]
+        public void StorageLoadStudySummaryTest(string path, string type)
+        {
+            IOptunaStorage storage = type == "sqlite"
+                ? new SqliteStorage(path)
+                : (IOptunaStorage)new JournalStorage(path);
+            StudySummary[] summary = Study.GetAllStudySummaries(storage);
+            Assert.Equal(3, summary.Length);
+            Assert.Equal(2, summary[0].Directions.Length);
+            Assert.Throws<InvalidOperationException>(() => summary[0].Direction);
+            Assert.Equal(StudyDirection.Minimize, summary[1].Direction);
+            Assert.Equal(3, summary[2].Directions.Length);
+        }
+    }
+}