A sealing apparatus for use in sealing a clearance between a cylinder (38) and a piston (50) without using a lubricating oil. An annular groove (66) is formed in the outer circumferential surface of the piston (50). First and second seal members (68, 70) formed out of a resin material are fitted annularly in an axially piled two-stage in this annular groove (66). A third seal member (72) is fitted annularly on the inner side of these first and second seal members (68, 70). On the inner side of the third seal member (72), a non-endless expander ring (74) adapted to press the first and second seal members (68, 70) against the inner circumferential surface of the cylinder (38) via the third seal member (72) is provided. The first, second and thrid seal members (68, 70, 72) are fitted in the groove (66) so that the clearances between both ends of these seal members are staggered from one another in the circumferential directions thereof and not aligned with one another, and, therefore, the three seal members (68, 70, 72) are arranged in the groove (66) as if a single endless ring were fitted therein.