PURPOSE: To facilitate exchange of consumable goods and treatment during the occurrence of abnormality through holding of a cover body in an arbitrary opening/closing position by a method wherein the weight of a balancer attached to a cover body is changed and regulated according to the change of a gas pressure and the characteristics of a spring.

CONSTITUTION: A cover body 3 is openably mounted on a fixed part 1 centering around a support shaft 2 and the cover body 3 is held in an arbitrary opening/ closing position against the weight of the cover body 3 through the force of a spring 4. In this case, a balancer 5 is attached to the cover body 3 and the weight of the balancer 5 is easily regulated through the change of the number of weights consisting of a number of plate bodies according to the change of a gas pressure and spring characteristics. A gas spring, a torsion bar, a twisted coil spring, and a leaf spring are usable as a position holding means 4. This constitution causes prevention of natural closing of the cover body 3 through lowering of a gas pressure and the change of spring characteristics owing to a change with a time and improvement of operability.