A cam mechanism has different portions thereof arranged on a main unit 1 and on a door 2. When the door 2 is closed, a hinge pin 23 is locked at one end of a hinge groove 9, keeping the cam mechanism in a first locked position. As the door 2 is opened, the hinge pin 12 is guided by the hinge groove 9 and a guide pin 25 is guided by a guide groove 11, permitting the door 2 to rotate and slide. Thus, the hinge pin 23 is locked at the other end of the hinge groove 9, bringing the cam mechanism to a second locked position. A boss 10 slides on a rib 19 and thereby maintains the second locked position, permitting the door 2 to rotate about the hinge pin 23. This structure helps realize a door opening/closing mechanism with enhanced operability at reduced cost.