In a method for protecting an electric wire used for supplying electric power from a power source to a load, conduction current (I(n)) supplied to the load is detected at every predetermined time interval (Deltat). Using the detected conduction current (I(n)), an increased temperature (DeltaTw(n)) of the electric wire is calculated by a relational equation between heat release and generation of the electric wire. The calculated increased temperature (DeltaTw(n)) is added to a reference temperature to estimate the temperature of the electric wire. When the estimated electric wire temperature is less than a predetermined upper limit temperature, the calculation of the increased temperature (DeltaTw(n)) is repeated. When the estimated electric wire temperature is greater than or equal to a predetermined upper limit temperature, power supply from the power source to the load is interrupted, so that the electric wire is protected.