To provide a cover opening/closing mechanism simply constructed to keep a cover plate in an opened condition, when opened.

A cover plate 3 is turnably supported at one end by a hinge mechanism 4. The hinge mechanism 4 comprises: a pair of bearing parts 72a and 72b provided at positions opposing each other across a dial operation part 14; a pair of right and left pivotal shafts 5a and 5b non-turnably and slidably supported in journal holes 36a and 36b opened to the upper end positions at both side ends of the cover plate 3; and springs 6a and 6b energizing the pivotal shafts 5a and 5b toward openings, respectively. The pivotal shafts 5a and 5b have round shaft portions 52 which are taper-shaped by tapered faces 53 and 53 directed to the opposite sides, the right and left bearing parts 72a and 72b have bearing holes turnably supporting the round shaft portions 52 of the pivotal shafts 5a abd 5b, and V-shaped grooves are formed in the hole bottoms of the bearing holes, where the tapered faces 53 and 53 of the round shaft portions 52 abut when the cover plate 3 is closed, respectively.