PURPOSE: To improve the operation of cover by providing self-closing power and self-opening power to the cover with the dead point angle of a spring as a boundary and actuating air damper mechanism from the angle beyond the dead point angle.

CONSTITUTION: When a stop-eject operating key 19 is moved in the D direction, a stop operating member slides to actuate a lock arm. When the lock arm turns a cover 2 up to where it goes over the dead point line a-a, a spring 12 actuates in the direction of opening the cover 2. When it is truned further, the gear 43a of an air damper mechanism 43 and a rubber sheet 42 begin to contact and a spring 44 causes both to make pressure contact. Rotating force is applied to both by the spring 12. Here, the gear 43a revolves, but as the cover 2 opens increasingly, the spring 44 is elongated, by which the clearance l2 is increased and the terminal gear is speeded up, and therefore its wind plate recieves air resistance and the acceleration of the gear 43a is thereby controlled. Hence, the cover 2 opens slowly in the H direction simply by slightly rotating the cover 2 from the a-a line. To close the cover 2, the operations take place reverse from the above, thus the operating characteristic of the cover is improved