A sliding portion between a seal lip (7) and the circumference of a shaft (3) is made of a sliding face (10) so that the facial pressure at the initial action time can be lowered to suppress the proceeding of the wear of the seal lip (7) thereby to elongate the lifetime of a sealing device (1). Moreover, screw ridges (12) are formed from an atmospheric side slope (11) to a sliding face (10) so that satisfactory sealing properties can be exhibited initially and endurably for a long time.