PURPOSE: To simplify the constitution of a high voltage part and to achieve miniaturization as a whole, by a method wherein the current of a high voltage current path is allowed to intermittently flow to a bypass current path at a definite interval by a switching means and detected by a current transformer.

CONSTITUTION: A diode CD is interposed to a high voltage current path 21 in a forward direction and a bypass current path 22 is intermittently connected to the diode CD in parallel by the switching of a transistor Q1. The switching of this transistor Q1 is controlled by the output pulse of a pulse generator 23 transmitted through a pulse transformer T3. A current transformer CT is interposed to the bypass current path 22 and generates a current corresponding to the switching of the transistor Q1 to supply the same to a resistor R2. The voltage generated in the resistor R2 is held as a sample by a sample holding circuit 24 due to a switch S and a condenser C. This held voltage is inputted to a display circuit 25 to be subjected to level display.