A power unit is provided that is not affected by a voltage fluctuation of a reverse flow-preventive diode provided in an output line thereof and that can provide a stable output controlled with a high accuracy. A first rectified diode 17 rectified and a first smoothing capacitor 18 smoothes the voltage switched by a transformer 14 to generate a DC voltage (VP) at an A point. A reverse flow-preventive diode 21 and an output resistor 22 are provided between the A point and a positive output terminal 23. The comparison/detection circuit 25 compares the anode and cathode potentials of the reverse flow-preventive diode 21 with each other. The comparison/detection circuit 25 supplies a differential amplifier 26 the anode or cathode potential whichever is lower.