To allow a fuel cap 10 to close an inlet port FNb of a filler neck FN at a small operating angle (approximately 90°) and to improve sealability with a gasket GS.

The fuel cap 10 has a cap side engaging portion 20a to be engaged with an opening side engaging portion FNc of the filler neck FN for sealing a space from the filler neck FN via the gasket GS. The opening side engaging portion FNc is formed to be inclined by a predetermined angle from the right-angled direction to an axial direction where the cap is closed. In addition, the cap side engaging portion 20a has a guide surface 20b which is engaged following the portion FNc by rotating the cap 10 while being inserted into the filler neck FN in a direction for closing. The guide surface 20b has a first slope 20c having a large gradient angle of α2 and a second slope 20d of α1 smaller than α2.