Abstract: A seal structure of the present invention is composed of a sliding member 2, a supporting member 1 which stores the sliding member 2 slidably, and a seal ring 5 provided between the sliding member 2 and the supporting member 1. The seal ring 5 is formed by a lip section 10 on which a pressure of a hydraulic operating fluid acts and a heel section 11 for supporting a working pressure. Since the seal structure is such that an annular pressure receiving area between an inner circumference and an outer circumference of the lip section 10 is smaller than an annular supporting area between an inner circumference and an outer circumference of the heel section 11, a pressure which acts on the seal ring 5 is reduced and deformation to a sliding surface side of the heel section 11 is prevented.