The present invention presents a motor drive device capable of detecting jamming accurately without falsely detecting jamming when a movable part contacts with other members. Jamming by a backrest is detected by judging whether the value of a period of one pulse P1 and a period of the next pulse P2 (P2/P1) integrated for 150 times (integrated value: 150) exceeds a specified threshold (160) or not. Accordingly, if the backrest rubs against a backrest of an adjacent seat, and the motor speed suddenly drops for a short period, such “warp” is not falsely detected as jamming, and only actual jamming can be detected securely.