A power supply device (10) for a high-frequency treatment instrument includes an output section (112), a characteristic detector (130), and a stop signal generator (170). The output section (112) outputs power having a first frequency supplied to the high-frequency treatment instrument. The characteristic detector (130) detects a characteristic signal generated before the power outputted from the output section becomes excessive, the characteristic signal having a second frequency higher than the first frequency and being included in a signal related to the power. The stop signal generator (170) outputs a stop signal to stop or reduce an output of the power from the output section (112) when the characteristic signal is detected.
