A protection circuit having: a connecting terminal which is connected to an external circuit and which inputs and outputs a charging/discharging current of a secondary battery to and from the external circuit; a voltage detection unit for detecting an output voltage of the secondary battery; a current detection unit for detecting a discharging current of the secondary battery; a power consumption calculation unit for calculating power consumption of the external circuit by multiplying a value of the output voltage detected by the voltage detection unit by a value of the discharging current detected by the current detection unit; and an external protection unit for executing external protection processing of protecting the external circuit when the power consumption calculated by the power consumption calculation unit exceeds a power threshold set to a value that is not less than a maximum value of power consumption of the external circuit.