An oil pump unit comprises a pump shaft and a pump housing which stores the pump shaft. A balancer unit comprises a pair of balancer shafts and a balancer housing which stores the balancer shafts. The pump housing and the balancer housing are coupled to each other at least in the vicinity of a periphery of the pump shaft, and at a coupling portion in the vicinity of the periphery of a power transmission shaft is formed a connection groove which connects the inside to the inside of the coupling portion. Accordingly, it can be retrained that the reliability of journal portions of the balancer shaft of the balancer unit and the like is deteriorated by dusts and the like contained in high-pressure oil from the oil pump unit in a balancer device of an engine.