To move a movable base to a desired direction by operating an operation lever, detaching the lock of a gas spring and continuously pressurizing the operation lever as it is.

A lock button 72 is projected from a rod 59, pushes up the operation lever 60 and obstructs the movement of the rod 59. When the knob 75 of the operation lever 60 is pushed down, a shaft rod 69 turns the operation lever 60 counterclockwise with the upper end of the rear oblong hole 65 of a supporting metal fitting 62 as a supporting point. Thus, since the lock button 72 of the gas spring 57 is pushed down, the lock mechanism is released. Simultaneously, the shaft rod 68 is separated from the upper end of the front oblong hole 64 of the supporting metal fitting 61 and abutted to a lower end, and by pushing down the operation lever 60 in a lower direction further, this mounting base 54 is moved in the lower direction. The vertical movement of the mounting base 54 is operated by relatively small force since a balanced state is attained by the gas spring 57.