Abstract: A self-opening mechanism for a cap is capable of maintaining a restoring force of an elastic member and easily and tightly locking the cap and also releasing the locking. The mechanism has an elastic member (4) that is attached to an inner side of a hinge (3), and also a stopper block (7) is provided in a direction in which the elastic member (4) deforms so that the elastic member (4) deforms to an S-shaped form. Engagement pieces (10, 10a) in the cap (2) side are inserted into and fixed between hooks (16, 16a) of a push piece (18) and a protruding engagement pieces (15, 15a) to lock the cap more tightly. As the hook (16, 16a) moves upward and moves away from the protruding engagement pieces (15, 15a) when the push piece (18) is pressed, the engagement pieces (10, 10a) are released and a cover (2) automatically opens due to the restoring force of the elastic member (4).