The invention provides an oil pump resonator in which various vibrations caused by pulsations that change in response to changes in oil pressure on a discharge port side can be attenuated by a resonator that comprises only one chamber, whereby the volume occupied by the resonator can be minimized. An oil pump in an engine, for feeding oil from a suction port to a discharge port through rotation of a rotor fitted in a pump housing, includes: a discharge flow channel communicating with the discharge port; a resonator comprising an introduction channel formed in the discharge flow channel and a chamber communicating with the introduction channel; and a piston having a leading end face section that makes up an inner wall face of the chamber, and reciprocating in response to pulsation changes. The piston slides so as to reduce the volume of the chamber as the frequency distribution of the pulsations becomes higher.