A power supply apparatus comprising: a piezoelectric transformer; a signal generation means configured to generate a signal to drive the piezoelectric transformer; and a detection means configured to detect an output voltage (Vout) of the piezoelectric transformer, wherein the signal generation means is configured to determine a frequency of the signal based on a feedback signal corresponding to the output voltage (Vout) detected by the detection means and a target voltage signal corresponding to a target voltage, characterized by: a setting means configured to set a variation amount of the frequency of the signal according to the target voltage, wherein in a case where the target voltage is a first voltage, the setting means sets a change amount of frequency of the signal to a first value, and changes the frequency of the signal per the first value in a period from a predetermined frequency to a target frequency corresponding to the first voltage; and wherein in a case where the target voltage is a second voltage less than the first voltage, the setting means sets a change amount of frequency of the signal to a second value larger than the first value, and changes the frequency of the signal per the second value in a period from the predetermined frequency to a target frequency corresponding to the second voltage; wherein the first value and the second value correspond to a setting value of a control parameter used to perform PID control; and further comprising a memory means configured to store a table in which gains each associated with each target voltage signal in the feedback control are stored, wherein the setting means is configured to refer to the table stored in the memory means, and to switch to one gain among the gains each associated with each target voltage. a piezoelectric transformer; a signal generation means configured to generate a signal to drive the piezoelectric transformer; and a detection means configured to detect an output voltage (Vout) of the piezoelectric transformer, wherein the signal generation means is configured to determine a frequency of the signal based on a feedback signal corresponding to the output voltage (Vout) detected by the detection means and a target voltage signal corresponding to a target voltage, characterized by: a setting means configured to set a variation amount of the frequency of the signal according to the target voltage, wherein in a case where the target voltage is a first voltage, the setting means sets a change amount of frequency of the signal to a first value, and changes the frequency of the signal per the first value in a period from a predetermined frequency to a target frequency corresponding to the first voltage; and wherein in a case where the target voltage is a second voltage less than the first voltage, the setting means sets a change amount of frequency of the signal to a second value larger than the first value, and changes the frequency of the signal per the second value in a period from the predetermined frequency to a target frequency corresponding to the second voltage; a setting means configured to set a variation amount of the frequency of the signal according to the target voltage, wherein in a case where the target voltage is a first voltage, the setting means sets a change amount of frequency of the signal to a first value, and changes the frequency of the signal per the first value in a period from a predetermined frequency to a target frequency corresponding to the first voltage; and wherein in a case where the target voltage is a second voltage less than the first voltage, the setting means sets a change amount of frequency of the signal to a second value larger than the first value, and changes the frequency of the signal per the second value in a period from the predetermined frequency to a target frequency corresponding to the second voltage; wherein the first value and the second value correspond to a setting value of a control parameter used to perform PID control; and further comprising a memory means configured to store a table in which gains each associated with each target voltage signal in the feedback control are stored, wherein the setting means is configured to refer to the table stored in the memory means, and to switch to one gain among the gains each associated with each target voltage.