A seal device is provided with: a seal ring (1) sliding on and in sealing contact with the surface of a shaft (A); a hollow partition wall (2) in contact with the seal ring (1); and a housing (B) for holding the seal ring (1) and supporting the hollow partition wall (2). The seal ring (1) consists of an elastic material and is provided with: a lip section (1a) in sliding contact with the surface of the shaft (A) over the entire circumference thereof; an elastic arm section (1b) supporting the lip section (1a) at the position of the sliding contact; and a key section (1c) connecting the elastic arm section (1b) and the housing (B). The constriction force acting on the shaft (A) is adjusted by the expansion and contraction of the hollow partition wall (2).