A high-voltage power unit capable of selectively providing a high voltage of positive or negative polarity, comprising: a positive voltage generating circuit for generating a positive high voltage between two output terminals (P1, P2); a negative voltage generating circuit for generating a negative high voltagebetween two output terminals (Q1, Q2); a control circuit for controlling each of the positive high voltage generated by the positive voltage generating circuit and the negative high voltage generated by the negative voltage generating circuit; wherein: each of the positive voltage generating circuit and the negative voltage generating circuit includes a booster transformer (T1, T2), and a rectifier circuit forming a Cockcroft-Walton circuit connected to a secondary winding of the transformer; characterized in that: one output terminal of the positive voltage generating circuit and one output terminal of the negative voltage generating circuit are connected together so as to connect the two voltage generating circuits in series connection; and a first resistor is connected between the two output terminals (P1, P2) of the positive voltage generating circuit; and a second resistor is connected between the two output terminals (Q1, Q2) of the negative voltage generating circuit; and one of two terminals of the series connection is a ground reference side, and a high-voltage output whose polarity can be changed is taken out from the other terminal; and the control circuit is adapted to control the positive voltage generating circuit and the negative voltage generating circuit so as to change the polarity of the high-voltage output of the high-voltage power unit. a positive voltage generating circuit for generating a positive high voltage between two output terminals (P1, P2); a negative voltage generating circuit for generating a negative high voltagebetween two output terminals (Q1, Q2); a control circuit for controlling each of the positive high voltage generated by the positive voltage generating circuit and the negative high voltage generated by the negative voltage generating circuit; wherein: each of the positive voltage generating circuit and the negative voltage generating circuit includes a booster transformer (T1, T2), and a rectifier circuit forming a Cockcroft-Walton circuit connected to a secondary winding of the transformer; each of the positive voltage generating circuit and the negative voltage generating circuit includes a booster transformer (T1, T2), and a rectifier circuit forming a Cockcroft-Walton circuit connected to a secondary winding of the transformer; characterized in that: one output terminal of the positive voltage generating circuit and one output terminal of the negative voltage generating circuit are connected together so as to connect the two voltage generating circuits in series connection; and a first resistor is connected between the two output terminals (P1, P2) of the positive voltage generating circuit; and a second resistor is connected between the two output terminals (Q1, Q2) of the negative voltage generating circuit; and one of two terminals of the series connection is a ground reference side, and a high-voltage output whose polarity can be changed is taken out from the other terminal; and the control circuit is adapted to control the positive voltage generating circuit and the negative voltage generating circuit so as to change the polarity of the high-voltage output of the high-voltage power unit. one output terminal of the positive voltage generating circuit and one output terminal of the negative voltage generating circuit are connected together so as to connect the two voltage generating circuits in series connection; and a first resistor is connected between the two output terminals (P1, P2) of the positive voltage generating circuit; and a second resistor is connected between the two output terminals (Q1, Q2) of the negative voltage generating circuit; and one of two terminals of the series connection is a ground reference side, and a high-voltage output whose polarity can be changed is taken out from the other terminal; and the control circuit is adapted to control the positive voltage generating circuit and the negative voltage generating circuit so as to change the polarity of the high-voltage output of the high-voltage power unit.