By using a target voltage Vc* of a capacitor connected to the output side of a DC / DC converter and a voltage Vb of a battery connected to the input side of the DC / DC converter, a duty ratio D as a drive instruction of the DC / DC converter is calculated. By using the voltage Vb, the electromotive force Vbo of the battery, and the charge / discharge current Ib of the battery, an internal resistance Rb is calculated. According to the internal resistance Rb and the electromotive force Vbo, the current value when the battery output becomes maximum is set as the upper limit value of the optimal current range IR, the DC / DC converter is driven / controlled by limiting the duty ratio D so that the current Ib is within the range of the optimal current range IR. Thus, it is possible to appropriately convert the battery input voltage.