An engine-driven generator G is concurrently used as a starter motor for an engine E that drives a working machine. A motor 25 for motive-power use is a driving source for the working machine to travel. The driving circuit 10 is commonly used between the starter motor G and the motor 25 for travel use. In a starting mode, with switches 35 and 36, the driving circuit 10 is connected to the starter motor G side to start the engine. After complete explosion of the engine, with the switches 35 and 36, the generator G is connected to a regulator 31, while the driving circuit 10 is connected to the motor 25 for travel use. As a result of this, while a battery 14 is electrically charged, the motor 25 for travel use is driven with the battery 14.