A first main back pressure supply port 55 and a first subsidiary back pressure supply port 56 are formed in a first sliding contact surface 5a of a first side member 5, the first main back pressure supply port 55 is formed in a groove shape in the first sliding contact surface 5a, and the first subsidiary back pressure supply port 56 communicates with the first subsidiary discharge port 54. A second main back pressure supply port 65 and a second subsidiary back pressure supply port 66 are formed in a second side member 6, the second main back pressure supply port 65 passes through the second side member in the axial direction and the second subsidiary back pressure supply port 66 is formed in a groove shape in a second sliding contact surface 6a.