A plug portion (27) is projected from a first block (1) so as to be inserted into a insertion hole (5) opened in a second block (2). A support surface (9a) is formed on the first block (1) so as to be brought into contact with a supported surface (2a) formed on the second block (2). In the support surface (9a) is opened a fluid flow hole (59). A gap supporting member (K) acting force upon the second block (2) in such a direction as to separate the second block (2) from the first block (1) so as to form a gap (G1) between the supported surface (2a) and the support surface (9a) is provided on the first block (1). A pull member (21) eliminates the gap (G1) by making the second block (2) approach the first block (1) against the force of the gap supporting member (K) in such a condition that an outer peripheral surface of the plug portion (27) is in close contact with an inner peripheral surface of the insertion hole (5) to bring the supported surface (2a) into contact with the support surface (9a).