FUEL TANK CAP

A fuel tank cap includes a seal cap body and a detaching-proof unit. The cap body has a handheld wall and a surrounding wall that extends from the handheld wall and that is formed with an engaging groove. The detaching-proof unit includes a ring sleeve member and a connection member. The ring sleeve member has an outer ring wall that surrounds the surrounding wall, and a resilient wall that extends the outer ring wall and that is engaged resiliently with the engaging groove. The connection member is connected to the ring sleeve member to be connected to a cap installation portion of a vehicle body.