To provide an opening/closing device having simplified lower-cost construction with a less number of parts and less assembling man-hours by improving the shape of a body case of a device body to omit a hinge case and a bracket.

An upward opening bottomed cylinder part 9 is integrally formed in part of the plastic body case 2 of the device body 1 and a pair of bearings 10, 11 opposing each other are integrally formed at the opening upper end of the bottomed cylinder part 9. A cam member 19 having a sliding protrusion 19a is pivotally mounted between the bearings 10, 11 rotatably around a hinge pin 12. A slider 17 having a cam abutting face 16 for abutting on the sliding protrusion 19a is housed in the bottomed cylinder part 9 slidably up and down. A spring member 18 is housed therein for energizing the slider 17 to be pushed up at all times. Since the slider 17 and the spring member 18 are housed directly in the bottomed cylinder part 9 integrally formed in the body case 2, assembling work can be done with a less number of parts and less assembling man-hours.