PURPOSE: To dispense with operator's selective operation and to prevent danger due to the overload of a crane by detecting variations in a lifting target of a lifting electromagnet on the basis of variations in magnetic flux flowing through magnetic poles of the electromagnet and by selecting excitation patterns of the electromagnet from detection results so as to be as required.

CONSTITUTION: The inward magnetic pole 8 of a lifting electromagnet is fitted with a search coil 9, and a fluxmeter 10, a voltage switching circuit 11, a signal generator 12 for forward excitation gate, and a reverse excitation signal generator 13 are provided. When steel susceptible to overlifting mixes in lifting targets and is attracted, it generates difference from these lifting materials and is detected through the fluxmeter 10 by variation in the induction power of the search coil 9 so as to automatically switch excitation patterns by the voltage switching circuit 11. Also it is so designed that upon ON command to the lifting electromagnet, excitation is made at a maximum voltage when the fluxmeter output is less than a prescribed value and excitation voltage is decreased when the fluxmeter output exceeds the prescribed value.