The power supply has a first power unit and second power unit, a control means for outputting the DC power to the DC supply line. A first power source unit, and performs the constant voltage (定 電壓) control, using a commercial power source (commercial power source). A second power supply unit, the output voltage monotonically (單調) is lowered as the output current increases, and a second power supply device for performing the tilt control of minor increase the output voltage as the output current decreases. And the control means, the optimal current value (the first when the conversion efficiency of the first power unit, the maximum value of the current to the first power supply unit is output to the DC supply line) measurement value than (the value of the current flowing through the DC supply line ) is greater when the second power supply unit is such that the output instruction value, the value of the current outputted to the DC supply line equal to the difference value between the measured value and the optimum current value. The second power source unit is set by changing the conditions of the slope control, without changing the output voltage value of the output current to a value corresponding to the instruction value.