Provided is an opening/closing device that includes a structure for inhibiting a brake engage part or a lock engage part from touching an object or the like and that prevent the perimeter of a guide rail from becoming thick due to the structure.In an opening/closing device in which a brake engage part is caused to engage with a first engaged part by a close prevention device to put a brake on the closing operation of an opening/closing body, a space adjacent to the horizontal width direction end face of the opening/closing body and continuous in the opening/closing body opening/closing direction is ensured, the space is encompassed in the opening/closing body opening/closing direction by the horizontal width direction end face of the opening/closing body, the guide rail, and the support member, and a detach spot for the first engaged part and the brake engage part is arranged within the space.