A high-voltage power unit (37, 38), comprising, for selectively outputting a high voltage of positive or negative polarity: a positive voltage generating circuit for generating a positive high voltage; a negative voltage generating circuit for generating a negative high voltage; and 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 one output terminal of a pair of output terminals (P1, P2) of the positive voltage generating circuit is connected to one output terminal of a pair of output terminals (Q1, Q2) of the negative voltage generating circuit to connect the two voltage generating circuits (2, 4) in a series connection, and one of two terminals of the series connection is used as a reference side, while the other terminal is used to take out a high-voltage output whose polarity is switched, characterised by the high-voltage power unit further comprising: a) a first impedance-variable circuit section connected between the pair of output terminals (P1, P2) of the positive voltage generating circuit; b) a second impedance-variable circuit section connected between the pair of output terminals (Q1, Q2) of the negative voltage generating circuit; c) a first output driver (60, 61, 66) for driving the second impedance variable circuit section to reduce an impedance of the second impedance-variable circuit section when a voltage or a current appearing at the pair of output terminals (P1, P2) of the positive voltage generating circuit exceeds a predetermined value; and d) a second output driver (63, 64, 67) for driving the first impedance-variable circuit section to reduce an impedance of the first impedance-variable circuit section when a voltage or a current appearing at the pair of output terminals (Q1, Q2) of the negative voltage generating circuit exceeds a predetermined value. a positive voltage generating circuit for generating a positive high voltage; a negative voltage generating circuit for generating a negative high voltage; and 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 one output terminal of a pair of output terminals (P1, P2) of the positive voltage generating circuit is connected to one output terminal of a pair of output terminals (Q1, Q2) of the negative voltage generating circuit to connect the two voltage generating circuits (2, 4) in a series connection, and one of two terminals of the series connection is used as a reference side, while the other terminal is used to take out a high-voltage output whose polarity is switched, characterised by the high-voltage power unit further comprising: a) a first impedance-variable circuit section connected between the pair of output terminals (P1, P2) of the positive voltage generating circuit; b) a second impedance-variable circuit section connected between the pair of output terminals (Q1, Q2) of the negative voltage generating circuit; c) a first output driver (60, 61, 66) for driving the second impedance variable circuit section to reduce an impedance of the second impedance-variable circuit section when a voltage or a current appearing at the pair of output terminals (P1, P2) of the positive voltage generating circuit exceeds a predetermined value; and d) a second output driver (63, 64, 67) for driving the first impedance-variable circuit section to reduce an impedance of the first impedance-variable circuit section when a voltage or a current appearing at the pair of output terminals (Q1, Q2) of the negative voltage generating circuit exceeds a predetermined value. a) a first impedance-variable circuit section connected between the pair of output terminals (P1, P2) of the positive voltage generating circuit; b) a second impedance-variable circuit section connected between the pair of output terminals (Q1, Q2) of the negative voltage generating circuit; c) a first output driver (60, 61, 66) for driving the second impedance variable circuit section to reduce an impedance of the second impedance-variable circuit section when a voltage or a current appearing at the pair of output terminals (P1, P2) of the positive voltage generating circuit exceeds a predetermined value; and d) a second output driver (63, 64, 67) for driving the first impedance-variable circuit section to reduce an impedance of the first impedance-variable circuit section when a voltage or a current appearing at the pair of output terminals (Q1, Q2) of the negative voltage generating circuit exceeds a predetermined value.