When a plasma is ignited in a plasma generator, an ion beam is made to run in the plasma generator, and in this state, a positive voltage with respective to ground is applied to a plasma production chamber from a DC power source. Secondary electrons are generated when the ion beam collides with a plasma generating gas which flows out of the plasma production chamber into a path of the ion beam. The secondary electrons are led into the plasma production chamber by the positive voltage, and within the plasma production chamber, a plasma ignition is triggered using the secondary electrons led into the plasma production chamber and a radio frequency.