PURPOSE: To miniaturize a detector and to reduce its price by detecting the change in magnetic flux in the slide direction of a pair of magnetic members where magnetic poles with mutually different gaps are provided, and obtaining a head position using a pair of detectors where a plurality of magnetoresistance effect elements are provided at an equal space.

CONSTITUTION: Magnetic members 1 and 2 are magnetized with N and S poles alternately in a slide direction X and N and S pole spaces being λ1 and λ2, respectively, and magnetoresistance effect MR elements 4-7 are provided at spaces of λ1/8 and λ2/8, respectively, at MR sensors 11 and 12 of a magnetic head 3. One terminal of the sensor 11 is connected to a power supply, the other terminal is connected to one terminal of the element 6, and an output terminal 8 is provided at a connection point, and the other terminal of the element 6 is grounded. The elements 5 and 7 are also connected and an output terminal 9 is provided. Similarly, output terminals 18 and 19 are provided for the sensor 12. Output signals Y11 and Y12 and Y21 and Y22 of the terminals 8 and 9 and 18 and 19 are as shown in expressions I and II and a subtraction value K of each multiplication value is as shown in an expression III. As a result, with the least common multiples of λ1/2 and λ2/2 as a measurement range, a position x can be detected.