Abstract: The object of the present invention is to improve sealing performance of a seal under a specific environment. The seal is arranged in an annular space A in which a rotary shaft 1 and a housing 2 face each other, and partitions the annular space in axial direction into a vacuum space B and an atmospheric pressure space C. The seal is constituted such that a main lip 6 of the seal has an axial contact width W0 with which the main lip slides on and contacts with the rotary shaft 1 and which is set to be equal to or larger than an axial contact width W1 necessary to secure minimum sealing performance with respect to a sealed portion and equal to or smaller than an axial contact width W2 with which sliding resistance against the rotary shaft becomes a permissible upper limit, to thereby prevent the axial contact width W0 from influencing rotation characteristics with respect to the rotary member and to allow securing required sealing performance.