Abstract: A flow passage coupling unit (1) for flow passages (2, 4) of movable and fixed members (3, 5), which includes a cylindrical body (6) secured to the movable member and having a large bore (10) communicating with the flow passage, a small bore (11) and a value seat (12) formed between the large and small bores; a sliding valve (7) having a cylindrical sliding surface (7a) which slidably fits in the small bore and has an axial passage (14), a valve head (17) movable within the large bore, and a through-hole (16) provided through the cylindrical sliding surface adjacent the valve head; a coil spring (13) provided within the large bore for biasing the valve head against the valve seat so that communication between the large bore and the axial passage is normally cut off; and the valve seat (12) made in a plane which is perpendicular to an axis of the sliding valve.