To firmly fix a top plate by providing an engagement member on the top plate or a supporting leg, providing a movable member on the other while turning and pressing it in an engagement direction and forming a first engagement edge by a contour to be abutted and engaged to the engagement member gradually tightly accompanying the turning in the movable member.

A fixing device 4 interposed between the top plate 2 and the supporting leg 3 is composed of a fixing plate 6 on the side of the supporting leg 3 and the movable member 7 on the side of the top plate 2 and an engagement pin 12 as the engagement member is attached to the fixing plate 6. First and second engagement edges 13 and 14 to be engaged and detached to/from a locking pin 12 are formed on the movable member 7 and they are formed so as to be respectively abutted to the engagement pin 12 gradually tightly accompanying the turning into the engagement direction of the movable member 7. As a result, when the top plate 2 is set at a using position and the engagement pin 12 is positioned inside the engagement groove 15 of the movable member 7, the top plate 2 is fixed to the using position without play. Also, the movable member 7 is pressed to the engagement direction by a helical spring 11 provided between a side frame 8 and surely held.