A method of operating a power source device for a high-frequency treatment instrument adapted to perform high-frequency treatment for biological tissue includes causing a high-frequency power source circuit to output power； setting a target impedance value which gradually increases from a change-over impedance value to a stop impedance value； regularly comparing a measured impedance value to the target impedance value； causing the high-frequency power source circuit to lower the power by a first ratio if the measured impedance value is greater than the target impedance value, and to raise the power by a second ratio if the measured impedance value is smaller than the target impedance value； and causing the high-frequency power source circuit to terminate the output upon the value for impedance reaching the stop impedance value.
