A circuit in a power supply unit, comprising: a first output terminal that outputs a voltage in a first polarity (Vc(-)) compared to ground; a second output terminal that outputs a voltage (Vb(+/-)) changeable between a voltage in the first polarity and a voltage in a second polarity, compared to ground, different from the first polarity; characterized by a transformer having primary winding, a first secondary winding and a second secondary winding; a first rectifying and smoothing circuit (D1, C1) that rectifies and smoothes an output of the first secondary winding and outputs a voltage in the first polarity to the first output terminal; a second rectifying and smoothing circuit (D2, C2) that rectifies and smoothes an output of the second secondary winding and outputs a voltage in the second polarity; first control means to which a PWM input signal is applied, the first control means including an operational amplifier, a power transistor and an RC circuit comprising a resistor and a capacitor and; second output control means for controlling the voltage (Vb(+/-)) that is output from the second output terminal, the second output control means including a series circuit of a fixed resistance part and a variable resistance part connected in series, wherein one terminal of the fixed resistance part is connected with an output terminal of the second rectifying and smoothing circuit (D2, C2) and the other terminal is connected to the second output terminal and wherein one terminal of the variable resistance part is connected to the first output terminal and the other terminal of the variable resistance part is connected to the second output terminal; and a control part that outputs a control signal for changing a resistance value of the variable resistance part; wherein the PWM input signal is smoothed by the RC circuit (R5, C3) and a feedback voltage of the voltage at the first output terminal are input to the operational amplifier and the output of the operational amplifier is supplied to the base of the power transistor and controls the current that flows through the power transistor so as to control the output voltage to be applied to the primary winding of the transformer. a first output terminal that outputs a voltage in a first polarity (Vc(-)) compared to ground; a second output terminal that outputs a voltage (Vb(+/-)) changeable between a voltage in the first polarity and a voltage in a second polarity, compared to ground, different from the first polarity; characterized by a transformer having primary winding, a first secondary winding and a second secondary winding; a first rectifying and smoothing circuit (D1, C1) that rectifies and smoothes an output of the first secondary winding and outputs a voltage in the first polarity to the first output terminal; a second rectifying and smoothing circuit (D2, C2) that rectifies and smoothes an output of the second secondary winding and outputs a voltage in the second polarity; first control means to which a PWM input signal is applied, the first control means including an operational amplifier, a power transistor and an RC circuit comprising a resistor and a capacitor and; second output control means for controlling the voltage (Vb(+/-)) that is output from the second output terminal, the second output control means including a series circuit of a fixed resistance part and a variable resistance part connected in series, wherein one terminal of the fixed resistance part is connected with an output terminal of the second rectifying and smoothing circuit (D2, C2) and the other terminal is connected to the second output terminal and wherein one terminal of the variable resistance part is connected to the first output terminal and the other terminal of the variable resistance part is connected to the second output terminal; and a control part that outputs a control signal for changing a resistance value of the variable resistance part; wherein the PWM input signal is smoothed by the RC circuit (R5, C3) and a feedback voltage of the voltage at the first output terminal are input to the operational amplifier and the output of the operational amplifier is supplied to the base of the power transistor and controls the current that flows through the power transistor so as to control the output voltage to be applied to the primary winding of the transformer.