To charge a work battery,too by using an alternator which follows vehicle standard without providing an additional alternator.

To charge a standard battery b with power of a standard alternator 28 by a sub battery charger (SBC) 45 and supplies power of the standard alternator 28 to a work device battery B side when voltage of the standard battery b reaches equal to or more than a predetermined voltage. A virtual power source 31b, obtained by converting voltage of the work device battery B, is provided on a charge device 31 for charging the work device battery B with power of the SBC 45, so as to become equal to the maximum voltage of the vehicle standard battery b.