PURPOSE: To detect abnormality in electric circuits by using a transformer which is provided on a power source base line with a detection gain adjustable freely by the number of turns as current detection sensor to detect circuit currents of different levels with one sensor accurately.

CONSTITUTION: Currents of different levels flowing through electric circuits connected to a power source base line 25 are detected as voltage accurately for each of the electric circuits according to a timing at which a load is controlled with a computer by means of a current detection sensor 22 made up of a transformer 26a with a detection gain adjustable freely by the number of turns, a Hall element 27 and the like. Then, abnormality in the electric circuits is judged by comparing the results of the detection with a reference voltage for each of the circuits.