Provided is an opening/closing device with a lock capable of performing the opening/closing and closing lock of a sliding door by a single actuator and arbitrarily setting the timing of a lock operation. In the opening/closing device (100) with a lock, an electric motor (301) is driven and the driving force of the electric motor (301) is input to the sun gear (310) of a planetary gear mechanism (300) and output from the internal gear (330) or the carrier (340). A pair of sliding doors (901, 902) are opened and closed by the driving force output from the internal gear (330). While the pair of sliding doors (901, 902) are at a fully closed position, the lock and lock-released states of a lock mechanism (600) are switched by a lock restricting mechanism (700).