PROBLEM TO BE SOLVED: To provide an assembly and construction method of an opening/closing device with which work to be implemented on a construction site of the opening/closing device can be reduced, work efficiency on the construction site can be improved and the work can be simplified.SOLUTION: A door body 1 of a sliding door device which is an opening/closing device, is guided by a slide rail 25 fixed to an upper frame member 10 which is a frame member, and moved for opening/closing. A slide rail fixing step for fixing the slide rail 25 through a bracket 17 to the upper frame member 10 is implemented e.g., in a factory prior to an upper frame member installing step for installing the upper frame member 10 on the construction site of the sliding door device. After the upper frame member installing step, a door body connecting step is implemented for connecting the slide rail 25 and the door body 1 via a connection member 20 coupled to the door body 1 on the construction site of the sliding door device.
