A pair of restraining plates are pivotally installed in a housing of a lock unit. The restraining plates have respective cam portions that are arranged to put therebetween a striker when the restraining plates are pivoted in respective given directions. A biasing member is arranged to bias one of the restraining plates to pivot in the given direction thereof, and a transmission mechanism is arranged between the restraining plates to induce a synchronized pivoting of the two restraining plates. Thus, when one of the restraining plates is pivoted in the given direction thereof, the other of the restraining plates is pivoted in the given direction thereof. With this arrangement, once the striker is locked by the lock unit, at least one of the cam portions of the restraining plates is pressed against the striker thereby to suppress or at least minimize undesirable play of the lock unit relative to the striker.