To reduce spike voltage generated while conducting switching between outputs of two DC-DC converters having different polarities of output voltage.

Relays R3, R4 for shorting part of primary windings L1 of two transformers 12, 22 are provided. The winding of the transformer 12 or 22 of the DC-DC converter 10 or 20 on the side where no output selection is made is shorted in association with relays R1, R2 for switching polarity of output voltage. As a result, an inducing action of voltage by magnetic coupling between the two transformers 12, 22 is restrained, so that charges can be prevented from gathering in a capacitor of a rectifying circuit 13 or 23 on the side where the DC-DC converter is in no action. Thus, the undesired spike voltage can be prevented from being outputted instantly at the time of switching the relay 2.