Seal ring and seal device

A seal device comprising:

• a shaft having an annular groove which is provided in an outer peripheral surface thereof and in which hydraulic pressure is introduced, and ring grooves each having side wall surfaces and communicating with the annular groove and provided on each side of said annular groove;
• a housing having an inner peripheral surface and a working chamber which communicates with the annular groove;
• seal rings, each being made of synthetic resin and having side surfaces and an outer circumferential surface, arranged in each ring groove in such a manner that said outer circumferential surface comes into sliding contact with the inner peripheral surface of said housing and one of said side surfaces comes into sliding contact with the side wall surface of said ring groove;
• and
• a piston reciprocated by being acted upon by hydraulic pressure within the working chamber;
• the side surfaces of each said seal ring being tapered such that the width of each seal ring on an inner circumferential side thereof is narrower than the width of the seal ring on an outer circumferential side thereof, the side surfaces being tapered at an angle of 2.degree. to 10.degree., and said outer circumferential surface having an arcuate cross section.