diff --git a/src/engine/candidate_list.h b/src/engine/candidate_list.h
index 08fa3d447..cb7979ca5 100644
--- a/src/engine/candidate_list.h
+++ b/src/engine/candidate_list.h
@@ -113,6 +113,15 @@ class CandidateList final {
         rotate_(rotate),
         focused_(false) {}
 
+  // Minimum copy operator.
+  // TODO(all): Support full copy operation of CandidateList as EngineConverter
+  // is currently copyable. Since the CandidateList contractor is
+  // initialized with `rotate`, `rotate_` is copied in the copy operator.
+  CandidateList &operator=(const CandidateList &candidate_list) {
+    rotate_ = candidate_list.rotate_;
+    return *this;
+  }
+
   void Clear();
 
   const Candidate &GetDeepestFocusedCandidate() const;
diff --git a/src/engine/engine_converter.cc b/src/engine/engine_converter.cc
index 545c3ec7e..58a31c3fa 100644
--- a/src/engine/engine_converter.cc
+++ b/src/engine/engine_converter.cc
@@ -1151,26 +1151,7 @@ void EngineConverter::FillOutput(const composer::Composer &composer,
 EngineConverter *EngineConverter::Clone() const {
   EngineConverter *engine_converter =
       new EngineConverter(converter_, request_, config_);
-
-  // Copy the members in order of their declarations.
-  engine_converter->state_ = state_;
-  // TODO(team): copy of |converter_| member.
-  // We cannot copy the member converter_ from EngineConverterInterface because
-  // it doesn't (and shouldn't) define a method like GetConverter(). At the
-  // moment it's ok because the current design guarantees that the converter is
-  // singleton. However, we should refactor such bad design; see also the
-  // comment right above.
-  engine_converter->segments_ = segments_;
-  engine_converter->incognito_segments_ = incognito_segments_;
-  engine_converter->segment_index_ = segment_index_;
-  engine_converter->previous_suggestions_ = previous_suggestions_;
-  engine_converter->conversion_preferences_ = conversion_preferences();
-  engine_converter->result_ = result_;
-  engine_converter->request_ = request_;
-  engine_converter->config_ = config_;
-  engine_converter->use_cascading_window_ = use_cascading_window_;
-  engine_converter->selected_candidate_indices_ = selected_candidate_indices_;
-  engine_converter->request_type_ = request_type_;
+  *engine_converter = *this;
 
   if (engine_converter->CheckState(SUGGESTION | PREDICTION | CONVERSION)) {
     // UpdateCandidateList() is not simple setter and it uses some members.
diff --git a/src/engine/engine_converter.h b/src/engine/engine_converter.h
index 4b9a957a0..1d33a8095 100644
--- a/src/engine/engine_converter.h
+++ b/src/engine/engine_converter.h
@@ -61,7 +61,6 @@ class EngineConverter : public EngineConverterInterface {
                   std::shared_ptr<const config::Config> config);
   explicit EngineConverter(std::shared_ptr<const ConverterInterface> converter);
   EngineConverter(const EngineConverter &) = delete;
-  EngineConverter &operator=(const EngineConverter &) = delete;
 
   // Checks if the current state is in the state bitmap.
   bool CheckState(States) const override;
@@ -245,8 +244,6 @@ class EngineConverter : public EngineConverterInterface {
   void OnStartComposition(const commands::Context &context) override;
 
   // Copies EngineConverter
-  // TODO(hsumita): Copy all member variables.
-  // Currently, converter_ is not copied.
   EngineConverter *Clone() const override;
 
   void set_selection_shortcut(
@@ -264,6 +261,9 @@ class EngineConverter : public EngineConverterInterface {
       std::numeric_limits<size_t>::max();
 
  private:
+  // Uses default copy operator to simply copy all members.
+  EngineConverter &operator=(const EngineConverter &) = default;
+
   friend class EngineConverterTest;
 
   // Resets the result value stored at the previous command.
diff --git a/src/engine/engine_converter_interface.h b/src/engine/engine_converter_interface.h
index d33e34507..3168300e5 100644
--- a/src/engine/engine_converter_interface.h
+++ b/src/engine/engine_converter_interface.h
@@ -66,10 +66,6 @@ struct ConversionPreferences {
 // support stateful operations related with the converter.
 class EngineConverterInterface {
  public:
-  EngineConverterInterface() = default;
-  EngineConverterInterface(const EngineConverterInterface &) = delete;
-  EngineConverterInterface &operator=(const EngineConverterInterface &) =
-      delete;
   virtual ~EngineConverterInterface() = default;
 
   typedef int States;