PURPOSE: To automatically and slowly open a cover 40 for closing an opening part 34 pierced on a top plate 32 of a desk 30 without generating a noise.

CONSTITUTION: A housing 44 is provided under an opening part 34, a pair of arms 50, 50 are attached so as to be freely oscillatable by oscillation shafts 52, 52 provided on both side walls 44a, 44a of the housing 44, and to its free end, a cover 40 is fixed so that it is oscillated by its dead weight and the opening part 34 is opened. A detaining mechanism 60 which protrudes and moves backward to and from the oscillation surface of the arms 50, 50 is provided so as to maintain a closed state of the cover 40 by detaining and controlling the oscillation of the arms 50, 50 caused by the dead weight. Also, tapered frictional resistance members 90, 90 whose thickness is large in the oscillating direction of the arms 50, 60 caused by the deat weight are provided on both side walls 44a, 44a of the housing 44, so that the arms 50, 50 oscillate, while executing a slide-contact.