PURPOSE: To achieve a good and stable switching operation continuously, by controlling time for feeding trigger current to a trigger electrode based on an output from a main current detecting section.

CONSTITUTION: Upon provision of an operation start command from outside to a command terminal 16, a control section 14 operates to feed trigger current IT from a trigger electrode 12 between a trigger electrode 7 and a main electrode 4b so as to conduct a pair of main electrodes 4a, 4b. Consequently, main current IM is fed from a main power source 10 to a load 13 then it is detected through a main current detector 15 and provided to the control section 14. The control section 14 controls the trigger power source 12 to keep operating condition so that the trigger current IT is fed continuously until the main current IM reaches a predetermined level IC. Since main electrodes 4a, 4b are conducting through low resistance, the main current IM increases even for a large inductive load and finally such condition as IM≥IC is brought out to provide stable discharge and establish conducting state between electrodes so as to stop operation of the trigger power source 12 through the control section 14. Thereafter, the main current IM stays in a stable discharge zone to maintain conducting state thus establishing switching operation.