A power unit includes: a frequency divider to divide a clock signal in a second frequency-division ratio based on a first frequency-division ratio and to output a driving pulse; a switching device driven by the driving pulse; a piezoelectric transformer to output an alternating-current high voltage when receiving an intermittent voltage from the switching device; a comparison device to compare a digital signal corresponding to the piezoelectric transformer output voltage with a target voltage and to output a comparison result; a controller to control the first frequency-division ratio based on the comparison result; a first holding device to hold the controlled first frequency-division ratio; a first computing device to determine the second frequency-division ratio by performing computation using a first correction value and the first frequency-division ratio held by the first holding device; and a second holding device to hold the second frequency-division ratio determined by the first computing device.