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； acquiring an initial impedance value in a first period from a start of the output； determining an increase rate of an output voltage relative to time； increasing the output voltage of the high-frequency electric power source circuit in accordance with the increase rate in a second period； acquiring the value relating to impedance of the biological tissue in the second period； and terminating the second period after the value relating to the impedance reaches a minimum value.
