An oil pump to be insatalled to an engine of an automotive vehicle. The oil pump includes a section defining a suction port and a section defining a discharge port. A main unit of the oil pump has a section defining a plurality of pump chambers. Volume of each pump chamber continuously changes to increase and decrease under driving of the engine so as to pressurize oil sucked through the suction port and discharge the oil through the discharge port. A section defining an oil chamber to which the oil flows is provided such that the oil chamber has a vertically upper side which is communicated with the discharge port through a communicating hole.