A variable displacement pump includes a housing having a pair of end wall surfaces; an annular outer rotor guide swingably disposed between the pair of end wall surfaces; a cylindrical outer rotor; an inner rotor provided radially inward of the outer rotor and configured to rotate integrally with a drive shaft at a location eccentric relative to the outer rotor; and a plurality of coupling plates coupling the inner rotor and the outer rotor. The outer rotor is rotatably fitted into an outer rotor supporting surface of the outer rotor guide. A space between the inner rotor and the outer rotor is partitioned into a plurality of chambers by the plurality of coupling plates. A concave portion is formed in the outer rotor supporting surface such that the concave portion exists over an entire axial range between the both end surfaces of the outer rotor guide.