To attain a power supply unit for field emission type electron gun, which can generate stable accelerating voltage by conducting feedback control stably and accurately even at low-accelerating voltage.

The ground side of a detection resistor 7 is floated, for example, at a high voltage of 1kV using a high-voltage power source (additional power source) 10 so that the load current of 1.5μA may flow through the detection resistor 7, even if accelerating voltage is 0.5kV. The current value flowing through the detection resistor 7 is converted into a voltage value, and transmitted to low-voltage side by an isolation amplifier 12. Although the output voltage of the isolation amplifier 12 and the reference voltage of a reference voltage source 9 are compared by a comparison amplifier 8, the detection resistor 7 is floated at a high voltage of 1kV, so that the load current of a Cockroft- Walton circuit 6 can be increased substantially. It is thus possible to maintain the output of the Cockroft-Walson circuit 6 at a prescribed voltage value by stable feedback control, even in the case of use in such a low-accelerating voltage condition as an electron returns from an anode 5 to an extracting electrode 3.