To easily mount a drain to the lower end of an inner door.

Grooves 21, 22 are formed at both faces of the inner door 5a near the lower end of the inner door 5a, and rails 23 inserted in the grooves 21, 22 are formed at the drain 19a of elastic structure, at the parts opposed to the grooves 21, 22 of the inner door 5a. A fastener such as a screw for fixing the drain 19a to the lower end of the inner door 5a is thereby dispensed with, and the drain 19a is easily mounted on the lower end of the inner door 5a. A conventional quality failure caused by the malfunction of the screw is reduced while improving productivity by reducing man-hours.