A processor of an electric power source device exercises control over a power output before a control switching time point based on a value related to initial impedance, and determines the control switching time point upon detecting that a value related to impedance of a living tissue is at a minimum. The processor sets a set electric-power value of the power output based on a parameter related to controlling the power output before the control switching time point, and performs constant-electric-power control by controlling the power output using the electric-power value at and after the control switching time point.
