PURPOSE: To prevent breakage of a device even when applying high voltage after shifting elements into inside of a vacuum by providing an output signal detecting means which gathers secondary electrons amplified by a secondary electrons doubling element to obtain an output signal and a power source voltage control means which controls the output voltage of a high voltage power source to apply voltage at the specified level to the said element.

CONSTITUTION: When creeping discharge occurs in an element 1, in the initial condition where secondary electrons doubling element 1 is shifted from the air to a vacuum, output signals of an output signal detecting circuit 12 rises up gradually, When the output signal exceeds the specified value, the detecting circuit 12 sends out an detection signal S to a power source voltage controller 3. The controller 13, when the signal S is inputted, stops the output voltage elevation of a high voltage power source 11. Hereby, the creeping discharge is calmed and the said output signal decreases gradually. When the output signal becomes the standard value, the output of signal S from the detection circuit 12 is stopped and the output voltage of the power source 11 rerises at the speed based on the time constant being set in advance by the controller 13. Thereafter, the similar action is repeated.