A case body is provided with a cover which opens from and closes against the case body. The cover includes a hinge boss rotatably attached to the case body through a hinge pin. On the opposite side of the hinge pin is provided a hook mechanism with which the cover is engaged with the case body to form a closed state. A pressure member contacts and presses to the hinge boss to reduce the rotation speed of the hinge boss. A radius reduction portion is formed so that the outer dimension of the hinge boss is reduced gradually as the cover is getting closer to its full-opened position in order to weaken a frictional contact function with the pressure member.