PURPOSE: To prevent the thermal deformation of a seal ring and maintain a favorable sealing effect by providing a stepped part on the inner wall face of said seal ring which is fitted the opening part of a cylinder body, engaging an auxiliary cylinder body on said stepped part, and pressing the projecting part of said auxiliary cylinder body with an annular elastic member.

CONSTITUTION: A cylinder member 23 is fitted in the support hole 21 of the side board 11b of a housing 11 which forms a base plate while a rotary shaft 13 is provided passing through the cylinder member 23. And, a support ring member 31 is fitted over the crank arm 13a of the rotary shaft 13 and a housing cylinder part 35 corresponding to a rotary cylinder body is integrally formed with one end of the support ring member 31. Further, a seal ring 51 is fitted in one end part of the housing cylinder part 35 while the seal ring 51 is pressed in the direction of the side board 11b by means of an annular spring 59. In this structure, a cut-off stepped part 56 is formed on the inner wall face of the seal ring 51 and an auxiliary cylinder body 58 is engaged with the cut-out stepped part 56. And, a slide ring plate 57 is bought into contact with the projecting part of the auxiliary cylinder body 58 to press the auxiliary cylinder body 58.