To provide a power circuit capable of switching over at a low voltage.

This power circuit is provided with a main power source 42 for electronic impact for applying an electron impact voltage in addition to a voltage due to an ionization power source 40 when degassing is performed by means of an electronic impact method. In addition, this main power source 42 for electronic impact is serially connected to the ionization power source 40. The circuit is also provided with a rectifying circuit 44 in which emission current is conductive. The rectifying circuit 44 is provided by directly connecting a secondary coil 50 and a diode 52 between a first terminal 46 and a second terminal 48. A smoothing capacitor 54 is provided between the first terminal 46 and the second terminal 48.