PURPOSE: To decrease the number of windings of the main circuit winding and the operating hysteresis width, by providing the auxiliary winding on the magnetic substance having the main circuit winding and turning on and off the current flowing to it in response to the magnetic flux through the magnetic substance.

CONSTITUTION: The main winding 1 connected in series to the main circuit through which DC flows, is wound on the magnetic substance 2, and Hall IC4 is inserted in the gap 3 of the magnetic substance 2. Further, when the magnetic flux density at the gap 3 is 1 or less, the transistot TR11 is OFF with the gap 3 is 1 or less, the transistor TR11 is OFF with the hysteresis characteristics of IC4 and TR12 is ON, the current I2 flows to the auxiliary winding 13 from the power supply 10, the current in the winding 1 increases. When the density is 1, TR11 is ON, TR12 is OFF. Thus, the detection display unit 14 is operated, and the current I2 is zero at the same time. Next, the current of the winding 1 decreases and the density is 2, then TR11 is OFF and TR12 is ON, to stop the display operation of the display unit 14.