Disclosed is an oil supply apparatus which can be realized at a low cost, and can reliably detect the lowered level of the oil filled in the oil reservoir unit. The oil supply apparatus is provided with an oil pump which is set to have a plurality of switching discharge pressures at which the oil discharge pressures are changed for each target rotation speeds when the rotation speed of the internal combustion engine reaches the respective target rotation speed. The ECU is adapted to determine that the level of oil stored in the oil pan is lowered under the condition that the deviation between the target rotation speed of the engine set to correspond to the switching discharge pressure and the rotation speed of the engine actually detected when the discharge pressure reaches the switching discharge pressure is not less than the determination value.