PURPOSE: To simplify structure and to eliminate the need for a button cover by engaging and rotating a cover body and a housing around a fulcrum whose sectional shape is determined by two tangents from one point at the outside of the circle and its circumference between the points of tangency.

CONSTITUTION: The sectional shape of one fulcrum around which the cover body 3 and housing of a tape recorder, etc., are rotated while engaged with each other is formed of the two tangents (l) and (l) drawn from one point P at the outside of the circle O to the circle and the circumference between the points of tangency. The other fulcrum consists of a curved line between two points P1 and P2 at the outside of the circle, straight line parts l1 and l1, and a circumferential part. Consequently, a opening and closing function with an angle Q is displayed sufficiently and the structure is simplified. This device is suitable for the cassette holder of the tape recorder.