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 a value 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; acquiring, based on the value relating to the output acquired from the output detection circuit, an initial impedance value which is a value relating to an impedance of the biological tissue in a first time period from a start of the causing step; 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 during a second time period following the first time period; acquiring a value relating to the impedance of the biological tissue during the second time period; and terminating the second time period after the value relating to the impedance reaches a minimum value, characterized in that the determined increase rate is dependent on the initial impedance value, such that the greater the initial impedance value is, the smaller the increase rate is, and in the increasing, the output voltage is increased linearly with time. 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 a value 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; acquiring, based on the value relating to the output acquired from the output detection circuit, an initial impedance value which is a value relating to an impedance of the biological tissue in a first time period from a start of the causing step; 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 during a second time period following the first time period; acquiring a value relating to the impedance of the biological tissue during the second time period; and terminating the second time period after the value relating to the impedance reaches a minimum value, characterized in that the determined increase rate is dependent on the initial impedance value, such that the greater the initial impedance value is, the smaller the increase rate is, and in the increasing, the output voltage is increased linearly with time. causing the high-frequency electric power source circuit to output the electric power; acquiring, based on the value relating to the output acquired from the output detection circuit, an initial impedance value which is a value relating to an impedance of the biological tissue in a first time period from a start of the causing step; 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 during a second time period following the first time period; acquiring a value relating to the impedance of the biological tissue during the second time period; and terminating the second time period after the value relating to the impedance reaches a minimum value, characterized in that the determined increase rate is dependent on the initial impedance value, such that the greater the initial impedance value is, the smaller the increase rate is, and in the increasing, the output voltage is increased linearly with time.