A power supply apparatus including a first circuit and a second circuit insulated from the first circuit includes an adjustment unit and a detector in the first circuit, a controller in the second circuit, and first and second communication units in the first and second circuits, respectively. The adjustment unit is configured to adjust power. The controller is configured to control the adjustment unit. The detector is configured to detect a parameter. The second communication unit is configured to perform wireless communication with the first communication unit. The first communication unit is operated with power supplied to the first communication unit by a signal generated in the first communication unit due to a signal output from the controller to the second communication unit, and transmits information about a result of detection by the detector to the second communication unit. The controller controls the adjustment unit based on the information.