An opening and closing apparatus with a lock for opening and closing a sliding door (11A,11B) that is installed at an entrance of a vehicle and is provided with an elastic member (12A,12B) at its door leading end, the opening and closing apparatus with a lock comprising: an actuator including an electric motor with an output shaft; a moving mechanism for moving the sliding door (11A,11B) in a predetermined opening direction (B1,B2) and closing direction (A1,A2), using output of the output shaft of the actuator; a lock mechanism that operates using output of the output shaft of the actuator and is capable of restricting movement of the sliding door (11A,11B) in the opening direction (B1,B2) when the sliding door (11A,11B) is at a fully closed position; and a control unit for controlling the actuator, wherein the lock mechanism includes an engaging member (66A,66B) capable of engaging with a lock member (14A,14B) capable of integrally moving with the sliding door (11A,11B), so as to restrict movement of the lock member (14A,14B) in the opening direction (B1,B2), and a restricting member capable of being displaced between a locking position for restricting displacement of the engaging member (66A,66B) when engaging with the lock member (14A,14B) and an unlocking position for allowing displacement of the engaging member (66A,66B), with an operation of the actuator, and the control unit controls the actuator such that a closing operation is performed, the closing operation serving as an operation of operating the moving mechanism so as to move the sliding door (11A,11B) along the closing direction (A1,A2) to the fully closed position and then displacing the restricting member from the unlocking position to the locking position, characterized in that at a predetermined intermediate time point during the closing operation, the predetermined intermediate time point being a time point when the remaining rotation amount for completing locking, calculated by the difference between the total rotation amount of the output shaft for completing the closing operation and the rotation amount of the output shaft, reaches a predetermined value, the control unit controls the actuator so as to reduce the output of the actuator from a predetermined first driving force to a second driving force, wherein the lock mechanism is operated by the second driving force. an actuator including an electric motor with an output shaft; a moving mechanism for moving the sliding door (11A,11B) in a predetermined opening direction (B1,B2) and closing direction (A1,A2), using output of the output shaft of the actuator; a lock mechanism that operates using output of the output shaft of the actuator and is capable of restricting movement of the sliding door (11A,11B) in the opening direction (B1,B2) when the sliding door (11A,11B) is at a fully closed position; and a control unit for controlling the actuator, wherein the lock mechanism includes an engaging member (66A,66B) capable of engaging with a lock member (14A,14B) capable of integrally moving with the sliding door (11A,11B), so as to restrict movement of the lock member (14A,14B) in the opening direction (B1,B2), and a restricting member capable of being displaced between a locking position for restricting displacement of the engaging member (66A,66B) when engaging with the lock member (14A,14B) and an unlocking position for allowing displacement of the engaging member (66A,66B), with an operation of the actuator, and the control unit controls the actuator such that a closing operation is performed, the closing operation serving as an operation of operating the moving mechanism so as to move the sliding door (11A,11B) along the closing direction (A1,A2) to the fully closed position and then displacing the restricting member from the unlocking position to the locking position, characterized in that at a predetermined intermediate time point during the closing operation, the predetermined intermediate time point being a time point when the remaining rotation amount for completing locking, calculated by the difference between the total rotation amount of the output shaft for completing the closing operation and the rotation amount of the output shaft, reaches a predetermined value, the control unit controls the actuator so as to reduce the output of the actuator from a predetermined first driving force to a second driving force, wherein the lock mechanism is operated by the second driving force.