PURPOSE: To prevent an upper cover from rapidly dropping by attaching tabs to an end of a stay and a lower base respectively and providing a bias spring for applying force to the end of the stay toward a spindle of the lower base.

CONSTITUTION: This cover opening/closing mechanism 3 has a bracket 6, a stay 7, a bias spring 8 and a braking spring 9. One of tabs of the bias spring 8 is attached to an end 7a of the stay 7 via a stud 8a, and the other tab is attached to a control unit via a stud 8b. A stud moving long hole 7c is formed between the end 7a and time other end 7b of the stay 7, and a moving stud 13 is slidably fitted into the hole. Thus the end 7b of the stay 7 is always pulled in a direction toward a spindle along the long hole of the lower base, so that even if a hand slips while an upper cover is being opened/closed, the upper cover may not collide with the lower base.