Abstract: A mounting groove of a seal structure using an O-ring is defined by: a groove bottom surface; an acute-angle-side groove side surface inclined at an acute angle contiguously to the radially inner edge of the groove bottom surface; and an obtuse-angle-side groove side surface inclined at an obtuse angle contiguously to the radially outer edge of the groove bottom surface. The O-ring is mounted in a state of being pushed and expanded to the acute-angle-side groove side surface. The O-ring is constrained, in the mounting groove, between the groove bottom surface and the acute-angle-side groove side surface. The O-ring is prevented from falling off from the mounting groove. A groove width is constant or increases from the groove bottom surface toward a groove opening. Work for mounting the O-ring on the mounting groove is easy, and processing for the mounting groove is also easy.