To provide a DC power supply unit that can supply power to a load without any troubles even if a supply voltage is large, and can reduce power consumption without any load.

This DC power supply unit is equipped with a transformer 4, a rectifying circuit 17 that uses voltage doubler rectification, places a transistor Q3 between a diode D2 and a smoothing capacitor C2 along with a low- voltage output terminal 17L, and can turn on and off the output of a high- voltage output terminal 17H, and a constant-voltage circuit 18 that stabilizes the voltage of the output with the higher voltage of the rectifying circuit 17. When a commercial AC supply voltage is higher than a specific value, the transistor Q3 is turned off, thus turning off the output of the high-voltage output terminal 17H, and selecting the low-voltage output terminal 17L of the rectifying circuit 17 by the constant-voltage circuit 18. On the other hand, when the commercial AC supply voltage is lower than the specific value, the transistor Q3 is turned on, thus turning on the output of the high-voltage output terminal 17H, and selecting the high-voltage output terminal 17H of the rectifying circuit 17 by the constant-voltage circuit 18 for making the voltage constant.