PURPOSE: To reduce the load of a worker and extend the life of a titanium filament by automatically switching the titanium filament to be electrified every electrifying cycle at the time of evacuation.

CONSTITUTION: When the switch of a control power source D is turned on, a timer circuit part 1 starts time measurement, a counter circuit part 2 automatically switches titanium filaments F1-F4 in order every cycle of the on/off of electrification to perform the connection. A current control part 5 controls to send a minute current to a selected filament F1-F4, in which the current is gradually raised to a current sent in a current setting part 4, then gradually reduced when an electrification cycle time set in the timer circuit 1 is passed, and lowered to a set minimum current. According to such a constitution, the adhesion of adsorptive materials on the surfaces of the filaments F1-F4 is reduced, and the current change at the time of switch of electrification and non- electrification is gentle, so that the lives of the filaments F1-F4 can be extended.