A load circuit protective device calculates increasing temperature of wire of a load circuit with a first predetermined sampling period (dt1) determined by a clock signal for normal operation when a semiconductor relay (Q1) is on to estimate the temperature of the wire. The temperature of the wire is therefore estimated with high accuracy. Moreover, when the semiconductor relay (Q1) is off, the load circuit protective device calculates decreasing temperature of the wire of the load circuit with a second predetermined sampling period (dt2) to estimate the temperature of the wire. The second predetermined sampling period (dt2) is determined by a clock signal for power-saving operation and is longer than the first predetermined sampling period (dt1). Accordingly, the calculation times of the temperature can be reduced, and the power consumption is therefore reduced.