A structure allowing a gasket to be mounted on a fluid device is provided. The gasket includes an annular attaching portion that has a first slope at an axial end. The fluid device includes an annular attached portion with a second slope that contacts the first slope. The first slope at least partially includes a contact area that contacts the second slope to seal a gap between the first slope and the second slope. The contact area has an annular bulge protruding toward the second slope in contrast to other portions of the first slope so that the bulge is the earliest contact portion with the second slope when the contact area is brought into contact with the second slope.