A coupling includes a coupling male having an engaging head and a coupling female having an engaged beam. The engaged beam is pushed and elastically deformed by the engaging head in a direction perpendicular to an inserting direction of the coupling male into the coupling female when the engaging head is engaging the engaged beam. It is not necessary to provide a pair of slits at the coupling male for causing the engaging head to engage the engaged beam. As a result, a length of the coupling male in the inserting direction of the coupling male can be made small. As a result, a thickness of the door apparatus can be made smaller than that of a conventional door apparatus.