PURPOSE: To make it possible to accumulate compressed air pressure utilizing the kinetic energy of a vehicle by turning on an electromagnetic clutch and controlling an air compressor so as to drive it when the pressure in the accumulator has not reached a given level at a vehicle speed above its lower limit.

CONSTITUTION: When the first piston 5 is lowered by a crankshaft through a connecting rod 6, an air compressor 31 takes air in the first cylinder chamber 4 and compresses it while the first piston 5 is rising to discharge it to a sub tank 1. Moreover, the compressed air is taken in the second cylinder chamber 2 while the second piston 3 is lowering, and is accumulated in an accumulator 11 compressed while the second piston 3 is rising. In this case, an electromagnetic clutch 18 is turned on by a controller 25 when the opening of an accelerator is smaller than the stationary opening of the accelerator at a vehicle speed and the pressure in the accumulator at a vehicle speed above a given lower limit, so as to carry out compression operation by transmitting the rotational force of a tire 19 to the connecting rod 6.