A power supply for an image forming device, comprising: a piezoelectric transducer having a prescribed resonant frequency and at least one spurious frequency (fs1, fs2) higher than the prescribed resonant frequency, for converting an input alternating current voltage at a variable frequency into a converted voltage; a driving circuit configured to generate the alternating current voltage input to the piezoelectric transducer that operates with a driving frequency equal to a frequency of the alternating current voltage; a voltage output unit configured to generate an output voltage from the converted voltage; a voltage detection unit configured to detect the output voltage and outputting a detected voltage value; and a frequency control unit (504, 508, 514) configured to control the driving frequency of the driving circuit by performing a digital operation on the detected voltage value; characterised in that the frequency control unit (504, 508, 514) is configured to vary the driving frequency in a first frequency range (Δ1) higher than the spurious frequency (fs1, fs2) and a second frequency range (Δ2) between the spurious frequency and the prescribed resonant frequency to make the output voltage track a target voltage, and configured to start the driving frequency at an upper limit of the first frequency range (Δ1), to decrease stepwise from the upper limit of the first frequency range (Δ1) and to change the driving frequency from the first frequency range (Δ1) to a first switchover frequency (fb)in the second frequency range (Δ2) when the driving frequency reaches a lower limit (fa) of the first frequency range (Δ1), thereby skipping over a prescribed frequency range including the spurious frequency (fs1, fs2). a piezoelectric transducer having a prescribed resonant frequency and at least one spurious frequency (fs1, fs2) higher than the prescribed resonant frequency, for converting an input alternating current voltage at a variable frequency into a converted voltage; a driving circuit configured to generate the alternating current voltage input to the piezoelectric transducer that operates with a driving frequency equal to a frequency of the alternating current voltage; a voltage output unit configured to generate an output voltage from the converted voltage; a voltage detection unit configured to detect the output voltage and outputting a detected voltage value; and a frequency control unit (504, 508, 514) configured to control the driving frequency of the driving circuit by performing a digital operation on the detected voltage value; characterised in that the frequency control unit (504, 508, 514) is configured to vary the driving frequency in a first frequency range (Δ1) higher than the spurious frequency (fs1, fs2) and a second frequency range (Δ2) between the spurious frequency and the prescribed resonant frequency to make the output voltage track a target voltage, and configured to start the driving frequency at an upper limit of the first frequency range (Δ1), to decrease stepwise from the upper limit of the first frequency range (Δ1) and to change the driving frequency from the first frequency range (Δ1) to a first switchover frequency (fb)in the second frequency range (Δ2) when the driving frequency reaches a lower limit (fa) of the first frequency range (Δ1), thereby skipping over a prescribed frequency range including the spurious frequency (fs1, fs2).