To provide a seal assembly that has a simple, low-cost, durable and efficient structure.

The seal assembly comprises an annular insert structure 24 of a rigid material, and an annular seal member 26 of an elastic material joined to the insert structure 24. The insert structure 24 of an I-shaped cross section has an outer annular flange structure 24a, an inner annular flange structure 24b concentric with the outer flange structure 24a, and an annular web structure 24c extending radially between both flange structures 24a and 24b. The web structure 24c defines, together with both flange structures 24a and 24b, a left and right annular grooves 30 and 32 continuous with left and right sides 24d and 24a of the insert structure 24, and defines a plurality of circumferentially spaced holes 24j extending axially through the web structure 24c or from one to the other of the left and right grooves 30 and 32. The seal member 26 fills both grooves 30 and 32 and the holes in the web structure 24c and extends to one side of the insert structure 24 to form an inner and outer annular sealing surfaces 26a and 26b separated from each other.