A power supply device for a high-frequency treatment instrument includes an output section, a characteristic detector, and a stop signal generator. The output section outputs power having a first frequency supplied to the high-frequency treatment instrument. The characteristic detector 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 outputs a stop signal to stop or reduce an output of the power from the output section when the characteristic signal is detected.
