portion 18 of the corresponding one of the balance shafts 13 l and 13 r the upper and lower housings 14 u and 14 l are put together as a result the two balance shafts 14 u and 14 l are rotatably retained between the two halves 14 u and 14 l of the balancer housing 14 thus the counter weights 19 are not required to be passed through any of the bearing bores and the diameters of the journal portions 18 and 19 can be reduced at will so long as the required mechanical strength is ensured therefore the rotational resistance can be reduced and the size and weight of the balancer housing 14 receiving the balance shafts 13 l and 13 r can be reduced to a level which has hitherto been impossible to achieve the front end of the balancer housing 14 or the lower half 14 l thereof is provided with a trochoid oil pump 27 for supplying lubricating oil to various parts of the engine as shown in fig6 also the trochoid oil pump 27 comprises a pump housing 28 attached to the front surface of the balancer housing 14 by threaded bolts an outer rotor 29 received in the pump housing 28 and an inner rotor attached to the front end of the right balance shaft 13 r the inner.