A seal device where a seal lip reliably maintains shaft sealing relative to high-pressure fluid that is to be sealed and where the equivalent Mises stress occurring in the seal lip is reduced to extend the durable term of the seal lip longer. A backup ring has a tapered section continuously extending from a fixation section of the backup ring and also has a forward end section. A rounded surface (E) with a curvature radius (R-1) is formed at least at a boundary where a surface (D) of the tapered section changes to a surface (F) of the forward end section. A portion of the surface (D) of the tapered surface, which portion is in contact with the rounded surface (E) at the boundary, is a projected curvature surface having a curvature radius greater than the curvature radius (R-1) of the boundary.