A variable displacement vane pump includes: an adapter ring (a cam ring housing member) that defines a first fluid pressure chamber and a second fluid pressure chamber relative to an outer periphery of a cam ring, the cam ring being moved relative to a rotor by a differential pressure between the first fluid pressure chamber and the second fluid pressure chamber; a seal housing groove formed in an inner periphery of the adapter ring; and a slipper seal which is inserted into the seal housing groove and with which an outer periphery of the cam ring comes into sliding contact when the cam ring moves, whereby the slipper seal partitions the first fluid pressure chamber from the second fluid pressure chamber, wherein the slipper seal is formed in a thin plate shape having a square cross-section.