A drive pulley (12) is formed by a piston-cylinder mechanism in which a seal ring (15) is set on an outer periphery of a fixed piston plate (16) that is in sliding contact with a cylinder inner peripheral surface (12d) of a cylinder (12b), and is driven by being supplied with hydraulic pressure. As a control parameter of a surface roughness shape of the cylinder inner peripheral surface (12d) of the drive pulley (12), a kurtosis (Rku) of a roughness curve and a skewness (Rsk) of the roughness curve, each of which is a height direction characteristic average parameter, are used. The cylinder inner peripheral surface (12d) is set to a surface having a surface roughness shape whose measurement values of the kurtosis (Rku) and the skewness (Rsk) are predetermined respective control values or less. With this, it is possible to surely suppress sliding abrasion of the liquid-tight seal member.