A caster mounting portion has a mounting member housing portion which houses the mounting member, the mounting member has a rotary shaft portion which is pivotally supported in a bearing recessed portion in the mounting member housing portion, an abutment body in which the rotary shaft portion is provided at an eccentric position and which is displaced by rotation to be widened to close the inside of the mounting member housing portion, an upper locking piece which engages with a locking receiving portion of an opposed wall, a lower locking piece which abuts against the bottom face of a latching portion, and a locking stepped portion which is urged by a leading end of an elastic arm, climbing over the aforementioned latching portion, and engages with the latching portion.