PURPOSE: To control an apparatus with a high accuracy, by connecting a negative power input terminal on the primary side with the negative terminal of a load and by inserting a load current-sensing resistance and a constant-voltage element into the feedback path for a load current for constant current- controlling said apparatus.

CONSTITUTION: A load 4 is supplied with power from a DC power supply via an oscillating transistor 1, a transformer 2, and a rectifier circuit 3. In the constant-current poewr circuit for said load, a load current-sensing resistance 5 and a constant voltage element 6 are inserted into the feedback path for a load current in parallel with each other. Consequently, both sensed voltage VA generated in said sensing resistance 5 and reference voltage VZ generated in said constant-voltage element 6 become negative voltages to GND, and a differential amplifier circuit 8 is connected with said parts 5, 6 to take an electric current ID according to the difference between both voltages VA, VZ. Thus, the electric current flowing into the constant-voltage element 6 is only a part of said load current, and the load current-sensing sensing resistance 5 forms a sensed voltage really based on said load current.