To provide an inexpensive opening/closing device which keeps an opening part of a bag body or the like from being opened easily by locking a slide member formed at a fastener part with a simple structure.

An opening/closing device 100 is formed on a fastener part 14 of a slide fastener, a plastic fastener or the like to make an opening part 12a of a case 10 of a bag body or the like made openable or closable. In the application, the opening/closing device 100 contains a slide 32 which is formed crossing over the fastener part 14 while being arranged free to slide and a fixing member 60 which is fastened at one end part in the longitudinal direction of the fastener part 14 to retain the slide member 32 in a state where the opening part 12a of the case 10 is closed. The slide member 32 and the fixing member 60 are made to butt together and under such a condition, the slide member 32 is retained on the fixing member 60 to lock the both, for example, by inserting and operating a key body 84 into a window part 80 of the fixing member 60.