To provide a seal structure preventing liquid leakage between a reciprocating operating shaft and a casing with high reliability not only in a liquid damper.

The operating shaft 12 is reciprocated in the casing 10 filled with liquid through an opening 10H, and a seal member 18 made of a soft member is provided for sealing liquid leaked out. The seal member is formed with an insertion hole 18H through which the operating shaft is inserted, and a plurality of annular projecting portions 18A, 18B having a projecting chevron-shaped cross section is formed on the wall of the insertion hole. The inside diameter of each annular projecting portion is smaller than the outside diameter of the operating shaft, and an inward slant surface K1 is inclined at a larger inclination angle than an outward slant surface K2. A seal area part 18J where the annular projecting portions are provided is so configured that it follows and is deformed with respect to the slight lateral vibration of the operating shaft by reducing the thickness of the seal area part thinner than that of an area where the annular projection portions are not provided.