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 two or more pressing areas, which are partitioned by annular steps. The second slope at least partially includes two or more pressed areas, which are partitioned by annular steps to fit the steps between the pressing areas.