The present invention relates to a tank cap including a lever member that is constrained from moving to the standing position contrary to the intention of the operator. The tank cap comprises a slide member disposed in the cap body so as to be adjacent to the lever member, and provided with a cam contact portion that is to come into contact with a cam portion of the lever member, and a coil spring disposed between the slide member and the bottom of the cap body. The resilience of the coil spring causes the cam contact portion to press the cam portion and retains the retracted position.