Use of a sealing device for partitioning an annular space formed between a rotary shaft and a housing surrounding the rotary shaft, into a low-pressure space and a high-pressure space whose pressure is higher than that of the low-pressure space, in an axial direction, the sealing device being mounted to a vacuum container whose interior is made into a vacuum environment, the low-pressure space being a vacuum space, the high-pressure space being an atmospheric pressure space, the sealing device comprising: an annular seal portion formed of an elastic material and being in slidable contact with an outer peripheral surface of the rotary shaft, wherein on an inner peripheral surface of the seal portion in a free state, a main lip is formed so as to be suitable for slidable contact with the outer peripheral surface of the rotary shaft, and an inclined surface is formed so as to extend from the main lip toward the low-pressure space side and gradually enlarge in diameter toward the low-pressure space side, and on the inclined surface, a plurality of annular projections are formed along the axial direction so as to be suitable for slidable contact with the outer peripheral surface of the rotary shaft; wherein the plurality of annular projections include a first annular projection provided at the main lip side, and a second annular projection provided at the low-pressure space side with respect to the first annular projection, the projection dimension of the second annular projection is larger than that of the first annular projection when the seal portion is in a free state, such that contact pressures of the first and second annular projections with respect to the outer peripheral surface of the rotary shaft become uniform in use, and the plurality of annular projections partition a space between the vacuum space and the atmospheric pressure space in multiple stages to reduce a negative pressure applied to the main lip, in a phased manner. an annular seal portion formed of an elastic material and being in slidable contact with an outer peripheral surface of the rotary shaft, wherein on an inner peripheral surface of the seal portion in a free state, a main lip is formed so as to be suitable for slidable contact with the outer peripheral surface of the rotary shaft, and an inclined surface is formed so as to extend from the main lip toward the low-pressure space side and gradually enlarge in diameter toward the low-pressure space side, and on the inclined surface, a plurality of annular projections are formed along the axial direction so as to be suitable for slidable contact with the outer peripheral surface of the rotary shaft; wherein the plurality of annular projections include a first annular projection provided at the main lip side, and a second annular projection provided at the low-pressure space side with respect to the first annular projection, the projection dimension of the second annular projection is larger than that of the first annular projection when the seal portion is in a free state, such that contact pressures of the first and second annular projections with respect to the outer peripheral surface of the rotary shaft become uniform in use, and the plurality of annular projections partition a space between the vacuum space and the atmospheric pressure space in multiple stages to reduce a negative pressure applied to the main lip, in a phased manner.