An opening/closing mechanism that allows a cover member to be biased according with the degree of opening. A solution is that an opening/closing mechanism of an opening/closing body, comprises: a cover member 15 opening and closing around a shaft center of a third shaft 33 and disposed on a base portion 23, and an opening/closing body 10 in which the cover member 15 is biased toward an opening direction by a biasing force of a spring member 21 wherein a spring support portion 35 is provided on the base portion 23, and a spring receiver 37 is provided on the cover member 15, and in accordance with more opening of the cover member 15, a distance from the spring base portion 41b of the spring member 21 to an abutting position in the spring extending portion 41a with the spring receiver 37 becomes longer and the biasing force decreases.