diff --git a/tool/lib/test/unit.rb b/tool/lib/test/unit.rb
index 01c53e929c..a748090b26 100644
--- a/tool/lib/test/unit.rb
+++ b/tool/lib/test/unit.rb
@@ -1409,14 +1409,16 @@ module Test
         if writer && test_path && status
           # Occasionally, the file writing operation may be paused, especially when `--repeat-count` is specified.
           # In such cases, we proceed to execute the operation here.
-          writer.write_object do
-            writer.write_key_value('testPath', test_path)
-            writer.write_key_value('status', status)
-            writer.write_key_value('duration', time)
-            writer.write_key_value('createdAt', Time.now.to_s)
-            writer.write_key_value('stderr', e)
-            writer.write_key_value('stdout', nil)
-          end
+          writer.write_object(
+            {
+              testPath: test_path,
+              status: status,
+              duration: time,
+              createdAt: Time.now.to_s,
+              stderr: e,
+              stdout: nil
+            }
+          )
         end
       end
 
@@ -1457,7 +1459,7 @@ module Test
           write_new_line
         end
 
-        def write_object
+        def write_object obj
           if @is_first_obj
             @is_first_obj = false
           else
@@ -1465,15 +1467,7 @@ module Test
             write_new_line
           end
           @indent_level += 1
-          write_indent
-          @file.write("{")
-          write_new_line
-          @indent_level += 1
-          yield
-          @indent_level -= 1
-          write_new_line
-          write_indent
-          @file.write("}")
+          @file.write(to_json_str(obj))
           @indent_level -= 1
           @is_first_key_val = true
           # Occasionally, invalid JSON will be created as shown below, especially when `--repeat-count` is specified.
@@ -1492,40 +1486,26 @@ module Test
 
         def write_array(key)
           @indent_level += 1
-          write_indent
           @file.write(to_json_str(key))
           write_colon
           @file.write(" ", "[")
           write_new_line
         end
 
-        def write_key_value(key, value)
-          if @is_first_key_val
-            @is_first_key_val = false
-          else
-            write_comma
-            write_new_line
-          end
-          write_indent
-          @file.write(to_json_str(key))
-          write_colon
-          @file.write(" ")
-          @file.write(to_json_str(value))
-        end
-
         def close
           return if @file.closed?
           close_array
           @indent_level -= 1
           write_new_line
-          @file.write("}")
+          @file.write("}", "\n")
           @file.flush
           @file.close
         end
 
         private
         def to_json_str(obj)
-          JSON.dump(obj)
+          json = JSON.pretty_generate(obj)
+          json.gsub(/^/, ' ' * (2 * @indent_level))
         end
 
         def write_indent
diff --git a/tool/test/testunit/test_launchable.rb b/tool/test/testunit/test_launchable.rb
new file mode 100644
index 0000000000..a91d44b1ce
--- /dev/null
+++ b/tool/test/testunit/test_launchable.rb
@@ -0,0 +1,57 @@
+# frozen_string_literal: false
+require 'test/unit'
+require 'tempfile'
+require 'json'
+
+class TestLaunchable < Test::Unit::TestCase
+  def test_json_stream_writer
+    Tempfile.create(['launchable-test-', '.json']) do |f|
+      json_stream_writer = Test::Unit::LaunchableOption::JsonStreamWriter.new(f.path)
+      json_stream_writer.write_array('testCases')
+      json_stream_writer.write_object(
+        {
+          testPath: "file=test/test_a.rb#class=class1#testcase=testcase899",
+          duration: 42,
+          status: "TEST_FAILED",
+          stdout: nil,
+          stderr: nil,
+          createdAt: "2021-10-05T12:34:00"
+        }
+      )
+      json_stream_writer.write_object(
+        {
+          testPath: "file=test/test_a.rb#class=class1#testcase=testcase899",
+          duration: 45,
+          status: "TEST_PASSED",
+          stdout: "This is stdout",
+          stderr: "This is stderr",
+          createdAt: "2021-10-05T12:36:00"
+        }
+      )
+      json_stream_writer.close()
+      expected = <<JSON
+{
+  "testCases": [
+    {
+      "testPath": "file=test/test_a.rb#class=class1#testcase=testcase899",
+      "duration": 42,
+      "status": "TEST_FAILED",
+      "stdout": null,
+      "stderr": null,
+      "createdAt": "2021-10-05T12:34:00"
+    },
+    {
+      "testPath": "file=test/test_a.rb#class=class1#testcase=testcase899",
+      "duration": 45,
+      "status": "TEST_PASSED",
+      "stdout": "This is stdout",
+      "stderr": "This is stderr",
+      "createdAt": "2021-10-05T12:36:00"
+    }
+  ]
+}
+JSON
+      assert_equal(expected, f.read)
+    end
+  end
+end