A protection apparatus for a load circuit is provided which can miniaturize electric wires and semiconductor switches by using a switch circuit simulating the current interruption time characteristics of fuses. A conductor resistance r and a heat resistance R used in each of a heat generation temperature calculation expression and a heat dissipation temperature calculation expression are respectively changed into a pseudo conductor resistance r* and a pseudo heat resistance R*, then the heat generation amount and the heat dissipation amount of the electric wire are calculated, and current temperature of the electric wire is estimated. When the estimation temperature reaches allowable temperature, an electronic switch S1 is turned off to thereby protect the load circuit. As a result, each of the electric wires and the semiconductor switches used in the load circuit can be protected from over heat.