A control device controls a surgical instrument including a heater. The control device includes a first power supply, a second power supply, a resistance value calculation section, and an output control circuit. The first power supply outputs a first current having a first frequency and supplied to the heater. The second power supply outputs a second current combined with the first current, supplied to the heater and having a second frequency different from the first frequency. The resistance value calculation section separates a signal component related to the second current from the first and second currents having passed through the heater, and calculates a heater resistance value based on the signal component. The output control circuit controls a temperature of the heater based on the heater resistance value.
