A maximum value selection unit (50) receives battery voltage values (Vb1, Vb2) and outputs the maximum value thereof to a lower limit value limiting unit (54). A maximum value selection unit (52) receives required voltage values (Vm1*, Vm2*) and outputs the maximum value thereof to the lower limit value limiting unit (54). The lower limit value limiting unit (54) outputs a voltage reference value (Vh*) by limiting the value so as not to fall below an output value of the maximum value selection unit (50). Switching commands (PWC1, PWC2) are generated based on a control computation using a combination of a voltage feedback control element and a voltage feedforward element, and a control computation using a combination of a current feedback control element and a voltage feedforward element, respectively.