To easily supply stable oil to hydraulic equipment or the like and prevent exertion of adverse effects on driving of the hydraulic equipment or the like by sufficiently reducing a pressure variation of the oil when moving the oil from an intake chamber to a discharge chamber and by reducing surging of a main gallery for feeding the oil to the intake chamber in a trochoid type oil pump.

An oil groove is formed in an oil pump casing or plate with which an inner rotor slidably contacts. One end side of the oil groove is communicatingly provided with a terminal end of the discharge chamber. The other end side of the oil groove is communicatingly provided with an oil chamber for the first time at a position where it is disconnected from the intake chamber and the discharge chamber.