Tank cap and tank cap apparatus

A fuel cap (110) closes the supply inlet (FNb) of a filler neck (FN) at a narrow operating angle (about 90 DEG ), improving the sealing properties of a gasket (GS). The fuel cap (110) has a cap engagement element (124), which is brought into engagement with the opening engagement element (FNc) of the filler neck (FN), and seals the gap around the filler neck (FN) by means of the gasket (GS). The opening engagement element (FNc) is formed in an inclined state at a predetermined angle relative to the direction orthogonal to the axial direction for closing the cap. The cap (110) has a tilt-preventing rib (129) formed on an outer circumferential portion of the cap (110) between the cap engagement element (124) and the gasket (GS), the tilt-preventing rib (129) being constructed and arranged to come into contact with an inner wall of the tank opening member (FN), thereby preventing the cap (110) from tilting upon application of a cap-tilting force.