A power supply apparatus supplies a power supply signal to a device's power supply terminal via a power supply line. An A/D converter receives, via a feedback line, an analog measurement value corresponding to the power supply signal supplied to the device's power supply line, and converts the analog measurement value into a digital measurement value. A digital calculation unit generates a control value by digital calculation such that the digital measurement value from the A/D converter matches a predetermined reference value. A D/A converter digital/analog converts the control value so as to supply the analog power supply signal to the device's power supply terminal via the power supply line. A feedback ratio calculation unit calculates the ratio between the control value and the digital measurement value. The digital calculation unit is configured to change its calculation content based on the ratio calculated by the feedback ratio calculation unit.