diff --git a/rdb/attrs.go b/rdb/attrs.go
index 8e8b74e9..24139039 100644
--- a/rdb/attrs.go
+++ b/rdb/attrs.go
@@ -8,8 +8,6 @@ import (
 // See https://github.com/c-bata/goptuna/issues/34
 // for the reason why we need following code.
 
-// Caution "_number" in trial_system_attributes must not be encoded.
-
 func encodeAttrValue(xr string) string {
 	return fmt.Sprintf("\"%s\"",
 		base64.StdEncoding.EncodeToString([]byte(xr)))
diff --git a/rdb/converter.go b/rdb/converter.go
index 57e9b058..fac264f5 100644
--- a/rdb/converter.go
+++ b/rdb/converter.go
@@ -2,8 +2,6 @@ package rdb
 
 import (
 	"errors"
-	"fmt"
-	"strconv"
 	"time"
 
 	"github.com/c-bata/goptuna"
@@ -21,13 +19,9 @@ func toFrozenTrial(trial trialModel) (goptuna.FrozenTrial, error) {
 
 	systemAttrs := make(map[string]string, len(trial.SystemAttributes))
 	for i := range trial.SystemAttributes {
-		if trial.SystemAttributes[i].Key == keyNumber {
-			systemAttrs[trial.SystemAttributes[i].Key] = trial.SystemAttributes[i].ValueJSON
-		} else {
-			systemAttrs[trial.SystemAttributes[i].Key], err = decodeAttrValue(trial.SystemAttributes[i].ValueJSON)
-			if err != nil {
-				return goptuna.FrozenTrial{}, err
-			}
+		systemAttrs[trial.SystemAttributes[i].Key], err = decodeAttrValue(trial.SystemAttributes[i].ValueJSON)
+		if err != nil {
+			return goptuna.FrozenTrial{}, err
 		}
 	}
 
@@ -52,15 +46,6 @@ func toFrozenTrial(trial trialModel) (goptuna.FrozenTrial, error) {
 		}
 	}
 
-	numberStr, ok := systemAttrs[keyNumber]
-	if !ok {
-		return goptuna.FrozenTrial{}, errors.New("number is not exist in system attrs")
-	}
-	number, err := strconv.Atoi(numberStr)
-	if err != nil {
-		return goptuna.FrozenTrial{}, fmt.Errorf("invalid trial number: %s", err)
-	}
-
 	state, err := toStateExternalRepresentation(trial.State)
 	if err != nil {
 		return goptuna.FrozenTrial{}, err
@@ -82,7 +67,7 @@ func toFrozenTrial(trial trialModel) (goptuna.FrozenTrial, error) {
 	return goptuna.FrozenTrial{
 		ID:                 trial.ID,
 		StudyID:            trial.TrialReferStudy,
-		Number:             number,
+		Number:             trial.Number,
 		State:              state,
 		Value:              trial.Value,
 		IntermediateValues: intermediateValue,
diff --git a/rdb/model.go b/rdb/model.go
index 8e1a7275..64a34c30 100644
--- a/rdb/model.go
+++ b/rdb/model.go
@@ -61,6 +61,7 @@ func (m studySystemAttributeModel) TableName() string {
 
 type trialModel struct {
 	ID               int        `gorm:"column:trial_id;PRIMARY_KEY"`
+	Number           int        `gorm:"column:number"`
 	TrialReferStudy  int        `gorm:"column:study_id"`
 	State            string     `gorm:"column:state;NOT NULL"`
 	Value            float64    `gorm:"column:value"`
diff --git a/rdb/storage.go b/rdb/storage.go
index a6949628..0f64f96b 100644
--- a/rdb/storage.go
+++ b/rdb/storage.go
@@ -2,7 +2,6 @@ package rdb
 
 import (
 	"fmt"
-	"strconv"
 	"time"
 
 	"github.com/c-bata/goptuna"
@@ -12,8 +11,6 @@ import (
 
 var _ goptuna.Storage = &Storage{}
 
-const keyNumber = "_number"
-
 // NewStorage returns new RDB storage.
 func NewStorage(db *gorm.DB) *Storage {
 	return &Storage{
@@ -234,12 +231,9 @@ func (s *Storage) CreateNewTrial(studyID int) (int, error) {
 		return -1, err
 	}
 
-	// Set '_number' in trial_system_attributes.
-	err = tx.Create(&trialSystemAttributeModel{
-		SystemAttributeReferTrial: trial.ID,
-		Key:                       keyNumber,
-		ValueJSON:                 strconv.Itoa(number),
-	}).Error
+	err = tx.Model(&trialModel{}).
+		Where("trial_id = ?", trial.ID).
+		Update("number", number).Error
 	if err != nil {
 		tx.Rollback()
 		return -1, err
@@ -313,9 +307,6 @@ func (s *Storage) CloneTrial(studyID int, baseTrial goptuna.FrozenTrial) (int, e
 
 	// system attrs
 	for key := range baseTrial.SystemAttrs {
-		if key == "_number" {
-			continue
-		}
 		err := tx.Create(&trialSystemAttributeModel{
 			SystemAttributeReferTrial: trial.ID,
 			Key:                       key,
@@ -364,11 +355,10 @@ func (s *Storage) CloneTrial(studyID int, baseTrial goptuna.FrozenTrial) (int, e
 		tx.Rollback()
 		return -1, err
 	}
-	err = tx.Create(&trialSystemAttributeModel{
-		SystemAttributeReferTrial: trial.ID,
-		Key:                       keyNumber,
-		ValueJSON:                 strconv.Itoa(number),
-	}).Error
+
+	err = tx.Model(&trialModel{}).
+		Where("trial_id = ?", trial.ID).
+		Update("number", number).Error
 	if err != nil {
 		tx.Rollback()
 		return -1, err
@@ -579,13 +569,11 @@ func (s *Storage) SetTrialSystemAttr(trialID int, key string, value string) erro
 
 // GetTrialNumberFromID returns the trial's number.
 func (s *Storage) GetTrialNumberFromID(trialID int) (int, error) {
-	var attr trialSystemAttributeModel
-	err := s.db.First(&attr, "trial_id = ? AND key = ?", trialID, keyNumber).Error
+	trial, err := s.GetTrial(trialID)
 	if err != nil {
 		return -1, err
 	}
-	number, err := strconv.Atoi(attr.ValueJSON)
-	return number, err
+	return trial.Number, err
 }
 
 // GetTrialParam returns the internal parameter of the trial