A seal structure is disclosed as mounted in a coupling 1, exposed in the vicinity of a stationary member 14, with a dust cover 15 being mounted to a cylindrical member 19 for preventing foreign material from entering a gap between the coupling 1 and the stationary member 14, and a seal member 17 is disposed at a position opposed to the dust cover on the stationary member 14, with a seal portion 123 of the seal 17 being held in sliding contact with the dust cover 15.