An operation method of an electric power source device for operating a high-frequency treatment instrument configured to perform a high-frequency treatment on a biological tissue includes causing a high-frequency electric power source circuit to output electric power; specifying an initial state of the biological tissue; acquiring a value relating to an impedance of the biological tissue; determining an additional impedance value based on the initial state; setting a stop impedance value which is the sum of the additional impedance value and a change-over impedance value; and causing the high-frequency electric power source circuit to stop the output, if the value relating to the impedance reaches the stop impedance value after the value relating to the impedance reached the change-over impedance value.