For a gear train GL including a drive gear 33 and an idler gear 34 engaged with each other and a lock gear 35, provided are a first drive means 3A configured to linearly drive the lock gear 35 in forward and backward directions, a second drive means 3B configured to rotationally drive the drive gear 33 in normal and reverse directions, and a controller C configured to control the both drive means 3A and 3B. The controller C starts driving the lock gear 35 at the time of an unlocking operation, from an engagement position toward the disengagement position through the first drive means 3A, and when the drive is started, the controller C drives the drive gear 33 into one of normal and reverse directions and into the other direction through the second drive means 3B with a polarity reversal in a predetermined cycles T1 and T2.