PURPOSE: To enable taking out of a large reproduced output with a smaller number of turn by providing a main magnetic pole, a coil winding the main magnetic pole and a transformer coupled magnetically to a part of the coil.

CONSTITUTION: A main magnetic pole 1 permits reduction in a relief 7 in a non- magnetic part in order to make a coil 1 of single turn. The magnetic flux passing through the pole 1 is thus increased and a voltage is generated efficiently in the coil 3 of single turn. Such voltage is increased by the coil 9 formed at the other end of the coil 3 and said voltage is utilized as a signal. A transformer 8 is adhered tightly to a base 5 and is made into the transformer of a closed loop having good efficiency as the magnetic flux passes through the base 5. The output is taken out from a part where the efficiency is good by forming the head into the single turn step-up type as mentioned above and therefore the number of turn is decreased. Since the transformer part having a large number of turn can be made into a separate body, the manufacture is made easy.