A power supply device for a high-frequency treatment instrument (41, 48) which performs high-frequency treatment on living tissue, the device comprising: an output section which outputs power having a first frequency supplied to the high-frequency treatment instrument (41, 48), wherein the device further comprises: a characteristic detector which 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; and a stop signal generator which has a timer for measuring a predetermined period of time of 50 milliseconds to 100 milliseconds and which outputs, for the predetermined period of time, a stop signal to stop or reduce an output of the power from the output section when the characteristic signal is detected, wherein the stop signal generator stops outputting the stop signal after outputting the stop signal for the predetermined period of time so that the output section resumes an output, the output section gradually increases the output after stopping the output for the predetermined period of time and resuming the output, so that the output section reverts to an output level before stopping the output, and the characteristic detector compares a first signal which is the signal related to the power and a second signal which is a signal related to the power that has passed a low-pass filter that passes a signal having a frequency lower than the second frequency and, when a signal value of the first signal becomes greater than a signal value of the second signal, the characteristic detector determines that the characteristic signal is detected. an output section which outputs power having a first frequency supplied to the high-frequency treatment instrument (41, 48), wherein the device further comprises: a characteristic detector which 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; and a stop signal generator which has a timer for measuring a predetermined period of time of 50 milliseconds to 100 milliseconds and which outputs, for the predetermined period of time, a stop signal to stop or reduce an output of the power from the output section when the characteristic signal is detected, wherein the stop signal generator stops outputting the stop signal after outputting the stop signal for the predetermined period of time so that the output section resumes an output, the output section gradually increases the output after stopping the output for the predetermined period of time and resuming the output, so that the output section reverts to an output level before stopping the output, and the characteristic detector compares a first signal which is the signal related to the power and a second signal which is a signal related to the power that has passed a low-pass filter that passes a signal having a frequency lower than the second frequency and, when a signal value of the first signal becomes greater than a signal value of the second signal, the characteristic detector determines that the characteristic signal is detected. a characteristic detector which 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; and a stop signal generator which has a timer for measuring a predetermined period of time of 50 milliseconds to 100 milliseconds and which outputs, for the predetermined period of time, a stop signal to stop or reduce an output of the power from the output section when the characteristic signal is detected, wherein the stop signal generator stops outputting the stop signal after outputting the stop signal for the predetermined period of time so that the output section resumes an output, the output section gradually increases the output after stopping the output for the predetermined period of time and resuming the output, so that the output section reverts to an output level before stopping the output, and the characteristic detector compares a first signal which is the signal related to the power and a second signal which is a signal related to the power that has passed a low-pass filter that passes a signal having a frequency lower than the second frequency and, when a signal value of the first signal becomes greater than a signal value of the second signal, the characteristic detector determines that the characteristic signal is detected.