To achieve a structure which fully secures the sealing performance by an auxiliary seal lip 16a during non-rotation and low speed rotation, concurrently fully reduces or loses the frictional force (rotational resistance) acting between a sliding contact rim 29 of the auxiliary seal lip 16a and an auxiliary seal sliding contact surface 19a during high speed rotation, and can fully ensure the seal life of the auxiliary seal lip 16a.

A plate-like connection part 26 without a constricted part in a part in a longitudinal direction thereof is provided in a base end part and an intermediate part of an auxiliary seal lip 16a, a thick-walled section 27 having a thickness size larger than the connection part 26 is provided similarly at a tip part thereof. Moreover, the sliding contact rim 29 provided at one edge in an axial direction of the thick-walled section 27 is brought into slide contact with the entire circumference of the auxiliary seal sliding contact surface 19a.