PURPOSE: To surely detect separation without being affected by the size of a steel plate to be suspended and without restriction on the place where separation state is not perfect by applying a voltage on a group of stacked steel plates in the stacking direction and by detecting presence of electric current flowing through the steel plate group.

CONSTITUTION: By using a direct current voltage generator 1, an electric current is guided to flow through an electromagnet 3 to a steel plate to be suspended and the steel plate remaining on the ground. A voltage drop generated by presence of the electric current is measured by a voltage measuring instrument 5 by inserting a resistor 2 large enough against a resistance value of the steel plate between the steel plate and the voltage generator 1 so as to constitute a circuit for returning the current from the lowermost part of the steel plate on the ground to the electric current generator 1 by means of a conductor 4. By detecting that the voltage level applied to the suspended steel plate is more than a predetermined threshold value, separation of steel plates is detected. The resistor 2 is to sufficiently reduce the voltage drop by the steel plate before separation and to enlarge the voltages before and after the separation.