To release a locked state without using a dedicated lock capable of selecting locking and unlocking modes by providing a casing with a lock device having a tongue piece to lock a latch rod at an engagement position.

When a door is closed, a latch rod 4 is pushed and energized with a spring 8, thereby coupling the end engagement part 4a of the latch rod 4 to a coupling hole 34 on an inner door support part 33. In this case, a stopper plate 9 is slightly project from the end engagement part 4a of the latch rod 4. In addition, the lower part of the end of the stopper plate 9 is sprung up, due to the lower end of the engagement hole 34 on the inner door support part 33 and enters the cutout part 35 thereof. In order to lock a door closed state subsequently, a screw driver is engaged with a tool insertion part 17 formed on the front of the turning shaft 12 of a locking device 11 and the turning shaft 12 is pushed down against the force of a spring. Furthermore, a tongue piece 13 is rotated until coming to a position faced to the base end of the latch rod 4. As a result, the retreat of the latch rod 4 is restricted and a knob 5 can be no longer made to slide. According to this construction, the latch rod 4 can be locked in an engagement state.