PURPOSE: To make a high voltage absorbing circuit unnecessary and to simplify a circuit, by changing the connection of a single-pole double throw switch for controlling the oscillation and stop of a DC-DC converter.

CONSTITUTION: A DC-DC converter 2 uses an NPN transistor TR6 as an oscillation switching element. In this state, when a contact 22c of a single-pole double throw switch 22 is switched to a contact point 22a, the base of the TR6 is connected to an oscillating electric power supply 1, also is connected to an earth line 19 of a high voltage circuit 3, and the converter 2 executes oscillation. Also, when the contact 22c is switched to a contact point 22b, the base of the TR6 is cut off from the electric power supply 1 and the earth line 19, also the earth line 19 is connected to a charging part A of the high voltage circuit, and the converter 2 stops its oscillation. In this way, since high voltage of the charging circuit A is applied nowhere, a high voltage absorbing circuit becomes unnecessary, and a circuit can be simplified.