A protection apparatus of a load circuit sets a threshold temperature at a lower temperature than an allowed temperature of an electric wire for use in the load circuit, and estimates a temperature of the electric wire based on an ambient temperature, a load current and a time while the load current is flowing through the electric wire. Then, in the case where the estimated temperature has reached the threshold temperature, a semiconductor relay (S1) is broken. As a result, in the case where such an electric wire temperature has risen owing to an occurrence of an overcurrent, and the like, the circuit is surely protected at the point of time before the electric wire temperature reaches the allowed temperature. Therefore, a fuse used in a conventional load circuit becomes unnecessary.