To suppress shocks applies to a lid body when it is opened from a casing.

A lid opening/closing device is equipped with a casing 4, a lid body 5 installed on the casing 4 for free opening and closing, and torsion springs 6A and 6B to generate energization in the direction of opening the lid 5 with respect to the casing 4. The lid body 5 has an arm 10 equipped protrusively with detent projections 12A and 12B in the direction of perpendicularly intersecting the rotating direction. The casing 4 is furnished with a stopper 7 to locate the opening position of the lid body 5 which is opened, and slide contact walls 15A and 15B are provided where the projections 12A and 12B on the arm 10 of the lid opened are in slide contacting.