To provide a cover mechanism for an opening/closing device, which prevents intrusion of dust from a clearance portion and which improves beauty.

A cover mechanism for an opening/closing device is provided in an opening/closing device 10 which includes a hinge arm 18 rotating to move a moving plate 14 between closing and opening positions and in which a clearance opening 7 for preventing interference with the hinge arm 18 in an opened state is formed in a second casing 3. The cover mechanism includes a cover 50 which closes the clearance opening 7 when the hinge arm 18 is located in a closing position and which is engaged with the hinge arm 18 in a opening position and urged for movement so as to be shifted from a position to close the clearance opening 7, and a torsion spring 60 for making the cover 5 urge in the direction of closing a clearance portion.