A control circuit controls power supplied via an electrical wire, by turning a FET provided on the electrical wire on/off. A control unit calculates the temperature difference between the ambient temperature of the electrical wire and the electrical wire temperature in time series, based on a previous temperature difference between the ambient temperature of the electrical wire 5 and the electrical wire temperature that was calculated previously, and the value of current flowing through the electrical wire. The control unit is configured to suspend this time-series calculation of the temperature difference, and clocks the elapsed time from suspension to resumption of calculation of the temperature difference. The control unit, in the case where calculation of the temperature difference is resumed, sets an initial temperature difference to be used as the previous temperature difference, based on the clocked elapsed time, in an initial calculation of the temperature difference after resumption.