A lock mechanism for a mobile member, comprising: a base; a mobile member movably supported to the base; and a lock device disposed between the base and the mobile member for locking the mobile member, the lock device comprising: a lock portion provided in either one of the base or the mobile member; a knob rotatably supported to the other of the base or the mobile member, and including a locking portion hooking to the lock portion on one end portion relative to a rotational axis of the knob, and an operating portion capable of releasing a locked state of the locking portion, which has been locked by hooking to the lock portion by rotating, and formed on the other end portion; and a return spring for urging the locking portion toward a direction of being hooked to the lock portion, wherein the lock device comprises: an elongate hole for movably supporting at least one end portion of the rotational axis along a moving direction of the mobile member; an urging device for urging the one end portion of the rotational axis toward a position of a front side of the elongate hole; and a rotation blocking portion for blocking a rotation of the knob when the one end portion of the rotational axis moves toward a deep side of the elongate hole against an urging force of the urging device. a base; a mobile member movably supported to the base; and a lock device disposed between the base and the mobile member for locking the mobile member, the lock device comprising: a lock portion provided in either one of the base or the mobile member; a knob rotatably supported to the other of the base or the mobile member, and including a locking portion hooking to the lock portion on one end portion relative to a rotational axis of the knob, and an operating portion capable of releasing a locked state of the locking portion, which has been locked by hooking to the lock portion by rotating, and formed on the other end portion; and a return spring for urging the locking portion toward a direction of being hooked to the lock portion, a lock portion provided in either one of the base or the mobile member; a knob rotatably supported to the other of the base or the mobile member, and including a locking portion hooking to the lock portion on one end portion relative to a rotational axis of the knob, and an operating portion capable of releasing a locked state of the locking portion, which has been locked by hooking to the lock portion by rotating, and formed on the other end portion; and a return spring for urging the locking portion toward a direction of being hooked to the lock portion, wherein the lock device comprises: an elongate hole for movably supporting at least one end portion of the rotational axis along a moving direction of the mobile member; an urging device for urging the one end portion of the rotational axis toward a position of a front side of the elongate hole; and a rotation blocking portion for blocking a rotation of the knob when the one end portion of the rotational axis moves toward a deep side of the elongate hole against an urging force of the urging device. an elongate hole for movably supporting at least one end portion of the rotational axis along a moving direction of the mobile member; an urging device for urging the one end portion of the rotational axis toward a position of a front side of the elongate hole; and a rotation blocking portion for blocking a rotation of the knob when the one end portion of the rotational axis moves toward a deep side of the elongate hole against an urging force of the urging device.