PURPOSE: To detect accurately the state of wear of a magnetic head, by detecting the wear progressing state of the head in the form a change of impedance of a magnetic head circuit.

CONSTITUTION: The impedance Z of a magnetic head circuit 1 increases as the depth length of a magnetic head 5 is reduced due to the wear of the head. As a result, the current and voltage supplied to the primary side of a rotary transformer 4 are varied, and an impedance detecting circuit VIMP goes up in accordance with the change of the depth length of the head 5. Therefore the input voltage V2 of a voltage comparator 8 rise higher than the reference voltage V1, and accordingly the light emitting part of a wear display device 10 extends toward scales t2, t3.... Then it is understood that the wear of the head 5 progresses up to the end of the head lifetime when the tip of the light emitting part of the device 10 reaches red scales t3 and t4. In such a way, the weared state of a magnetic head can be accurately detected.