PURPOSE: To enhance the durability and reliability of an oil pump in an engine incorporating a transmission mechanism for transmitting a drive power of a crank shaft to a balancer shaft by way of an intermediate idler to which the oil pump is coupled, by enhancing the supporting rigidity of the oil pump while keeping the engine in a compact lay-out so as to restrain vibration transmitted from the balancer shaft 4, from affecting upon the oil pump.

CONSTITUTION: With the use of a cover member 16 fixed to the wall surface of a cylinder block 1 of an engine, for covering a transmission mechanism 10, and a housing constituting part 19 adapted to be located between an intermediate idler 6 and the wall surface of the cylinder block 1, for forming a housing for an oil pump 11 is integrally incorporated with a part of the cover member 16. With the housing constituting part 19 attached to the wall surface of the cylinder block 1 increases the rigidity of the pump housing.