To eliminate a sliding surface pressing-down thrust possessed by an axial seal or to reduce the thrust up to a microscopic residual value by reducing a leakage loss by clearance by forming the seal sliding surface as the axial seal capable of close contact, and balancing oil pressure on the sliding surface to increase a speed.

A seal sliding ring 87 which is concentric with the axis arranged on a second shaft 15 and has an annular vertical plane, is arranged on the rotary axis to communicate/connect a fluid passage of two sets of rotary port grooves 27 existing on the rotatable second shaft 15 with/to two sets of external connecting ports 81 arranged in a casing 70 resting so as to be oiltightly freely slidable. An axial seal piston 86 which is concentric with the axis and has the annular vertical sliding surface, is symmetrically arranged in the shaft direction of the rotary port grooves 27 by firmly installing/fixing this seal sliding ring 87 on/to the second shaft 15. This mechanism is formed as a structure of pressing down the sliding surface by self-pressure acting by being slidably installed in the seal sliding ring direction opposed to a seal cylinder concentric with the outward opening rotary axis.