A high-voltage power source for applying high voltage to a nozzle of an ESI ion source includes a charge release assistant section including switch circuits and other elements for forcing electric charges accumulated at output terminals to be discharged in a polarity-switching operation, whereby the positive/negative switching of the polarity of the output voltage can be quickly performed. For example, when the voltage applied to the nozzle needs to be changed from V1 to V2 (where V1 and V2 are positive, and V1>V2), a voltage control section operates a positive voltage generation section and negative voltage generation section so as to temporarily provide a negative output voltage. After a predetermined period of time, the voltage control section operates the positive voltage generation section and negative voltage generation section so as to provide voltage V2.