To reduce size of a whole of a variable displacement pump by constituting it as a balanced type and reducing size of a pump rotary shaft and bearings.

A pair of projections 20, 20 projecting in positions facing each other at an inner periphery wall of an adapter ring 19 fittingly inserted into a pump body are provided. A cam ring 17 including grooves 21, 21 selectively engaged to the projections 20, 20 in an outer periphery and forming a roughly elliptical shaped cam face 17a in an inner periphery, is provided so as to rotate in a range of a predetermined angle inside the adapter ring 19. By pivotally supporting a rotor 15 with a vane 15a inside the cam ring 17, a pair of pump chambers 18A, 18B are formed at an axisymmetrical positions between the cam face 17a and the rotor 15. Intake side openings 28A, 28B and discharge side openings 27A, 27B are opened at different positions in the pump chambers 18A, 18B. Pressure controlling means 40, 41 guiding pressure for rotating the cam ring 17 are provided at operation chambers 22, 22 formed by one side parts 20b, 20b of the projections 20, 20 and the grooves 21, 21.