An annular seal member 2 is formed by overlapping, in the axial direction D1, a plurality of ring-shape parts 3 which are closed or unclosed when viewed in a radial direction D2 orthogonal to the axial direction D1 of the annular seal member 2. A seal groove 5 has a groove bottom 51 with an outer diameter R51 that increases along the axial direction D1. The inner periphery 22 of the annular seal member 2 contacts the groove bottom 51, and the inner diameter R22 of the inner periphery 22 increases as the outer diameter R51 of the groove bottom 51 increases. By this means, the outer diameter R21 of the outer periphery 21 increases, and the side of the outer periphery 21 of the annular seal member 2 that has a smaller outer diameter R21 does not contact the inner peripheral wall 61 of an inner peripheral wall member 6, and the side of the outer periphery 21 of the annular seal member 2 that has a greater outer diameter R21 does contact the inner peripheral wall 61 of the inner peripheral wall member 6.