To reduce cost by simplifying sealing structure.

A thin steel plate 21a is interposed between an end surface of a cam ring 1 and an end surface on the cam ring side of a side block 3, and rubbers 21b, 21c are adhered on marginal parts of both end surfaces of the thin steel plate 21a. As a strong sealing function is displayed by elastic deformation of the rubbers 21b, 21c, a complicatedly shaped O ring groove and an O ring are eliminated. As the thin steel plate 21a is used as a base material of a gasket 21, it is not necessary to apply Teflon coating to a rear side end surface of the front side block 3 and a front side end surface of a rear side block.