A cap includes a main body attached to an end part of a charging port for fuel gas. The main body includes an elastic deformable portion for locking the main body to the end part when the main body is attached to the end part. The cap further includes a cover covering at least part of the elastic deformable portion and surrounding the main body. The elastic deformable portion at least partially elastically deforms when the main body is attached to the end part and when the main body is detached from the end part. The cover is a separate member from the main body. When the elastic deformable portion elastically deforms, the cover applies force to the elastic deformable portion in a direction opposite from the deforming direction.