An electric power source device for operating a high-frequency treatment instrument configured to perform a high-frequency treatment on a biological tissue, the device comprising: a high-frequency electric power source circuit configured to output electric power; an output detection circuit configured to detect the output; and a control circuit configured to acquire information relating to the output from the output detection circuit, and configured to control an operation of the high-frequency electric power source circuit, the control circuit being configured to execute: causing the high-frequency electric power source circuit to output the electric power; specifying an initial state of the biological tissue in a first period from a start of the output; acquiring a value relating to an impedance of the biological tissue, after specifying the initial state 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, where the change-over impedance value is a value relating to the impedance indicative of a predetermined state; 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, wherein the specifying the initial state of the biological tissue includes specifying an initial impedance value which is the value relating to the impedance of the biological tissue acquired based on the output, and the additional impedance value is determined based on a prestored table in which the initial impedance value, a length of a second period and the additional impedance value are associated, where the second period is the period until the value relating to the impedance reaches the change-over impedance value after specifying the initial state. a high-frequency electric power source circuit configured to output electric power; an output detection circuit configured to detect the output; and a control circuit configured to acquire information relating to the output from the output detection circuit, and configured to control an operation of the high-frequency electric power source circuit, the control circuit being configured to execute: causing the high-frequency electric power source circuit to output the electric power; specifying an initial state of the biological tissue in a first period from a start of the output; acquiring a value relating to an impedance of the biological tissue, after specifying the initial state 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, where the change-over impedance value is a value relating to the impedance indicative of a predetermined state; 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, wherein the specifying the initial state of the biological tissue includes specifying an initial impedance value which is the value relating to the impedance of the biological tissue acquired based on the output, and the additional impedance value is determined based on a prestored table in which the initial impedance value, a length of a second period and the additional impedance value are associated, where the second period is the period until the value relating to the impedance reaches the change-over impedance value after specifying the initial state. causing the high-frequency electric power source circuit to output the electric power; specifying an initial state of the biological tissue in a first period from a start of the output; acquiring a value relating to an impedance of the biological tissue, after specifying the initial state 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, where the change-over impedance value is a value relating to the impedance indicative of a predetermined state; 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, wherein the specifying the initial state of the biological tissue includes specifying an initial impedance value which is the value relating to the impedance of the biological tissue acquired based on the output, and the additional impedance value is determined based on a prestored table in which the initial impedance value, a length of a second period and the additional impedance value are associated, where the second period is the period until the value relating to the impedance reaches the change-over impedance value after specifying the initial state.