While the amount of stored electricity is maintained at about 50 percent of a charging capacity using a direct-current power assist device connected to a direct-current circuit of an inverter, a certain period of time is monitored. When electricity is charged or discharged, the amount of peak cut, as a power assist, cannot be controlled. Moreover, it is impossible to carry out charge-and-discharge discrete control unique to a load corresponding to the intended use. In a setting section of a chopper control section of the direct-current power assist device, a charge start voltage, a charge stop voltage, a non-control voltage range, a discharge stop voltage, and a discharge start voltage are set as setting values. Each setting value is selected in accordance with a detection value of a direct-current detection voltage of the inverter, and output to a charge control section and a discharge control section as a charge target value and a discharge target value, respectively. Moreover, a load compensation gain section is provided to calculate a gain compensation section corresponding to a deviation signal between a terminal voltage discharge threshold and a terminal voltage of a capacitor device detected. Using the gain compensation signal, a limiter value of a driving torque limiter provided in a control circuit of the inverter is varied to adjust an instruction of current.