PURPOSE: To obtain a signal having an excellent S/N ratio by installing a first electromagnetic conversion element reading magnetism on a magnetic recording body and a second electromagnetic conversion element at a position being not subject to the effect of magnetism on the magnetic recording body and mounting a comparator outputting the differential section of both output values as an electric signal.

CONSTITUTION: A first MR element 1 is arranged near a position, where a magnetic recording body is passed, and a second MR element 2 is disposed at a position being not subject to the effect of the magnetism of the magnetic recording body in the vicinity of the element 1. The element 1 is connected to a constant current circuit 5a, constant currents Is1 flow, and voltage corresponding to the impedance of the element 1 is generated at a point (a). An AC section is extracted by a coupling capacitor 3 at a voltage value displayed at the point (a), and the value is inputted at the + terminal of a differential amplifier 6. The element 2 is connected to a constant current circuit 5b, and controlled so that constant currents Is2 flow, a value corresponding to the impedance of the element 2 is displayed at a point (b), and the AC section is extracted by a coupling capacitor 4 at the voltage value and input at the terminal of the amplifier 6. The differential section of the voltage values (a) and (b) is amplified by the amplifier 6, and a signal being not subject to the effect of noises is output to an outputted terminal (c).