When a trigger discharge between a metal cathode and a trigger ring induces a vacuum-arc discharge between the cathode an anode, which vaporizes the substances of the cathode surface to produce a metal ion plasma, setting the pulse length of the arc pulse applied between the cathode and the anode to 1 msec or longer will soon short-circuit between the cathode and the trigger ring due to the vaporized substances deposited on the surface of the insulating ring. In order to solve this problem, a permanent magnet 36 for forming a magnetic field across a space between the anode 26 and the cathode 34 is provided close to the rear side of the trigger ring 35 so as to guide the substances vaporized from the cathode 34 toward the anode 26. Thereby, a longer continuous operation can be done with setting the arc pulse longer. moreover, since the permanent magnet 36 is provided at the foregoing position, the magnet does not receive a thermal load by the vacuum-arc discharge, which maintains a stable operation and makes the total construction compact.