To provide a rotating speed sensor, which uses a magneto-resistive sensor, which is capable of appropriately preventing output signals decreasing to the relative displacement between a magnet and a magneto-resistive element.

A magnetic rotator 24 is polarized so that the direction of polarization can be inverted at every circumferential predetermined angle. GMR elements 36 and 38 are arranged so as to sandwich a magnet rotor 24 from both sides. When the magnet rotor 24 rotates, the magnetic filed acting on the GMR elements 36 and 38 varies periodically. When the magnet rotor 24 is axially displaced, since the amt. of change in the resistance of one GMR element decreases and the resistance of the other increases, the increase and decrease in the resistances offset each other by taking series resistance. Therefore, the decrease of the amplitude in the output signal caused by these displacement is prevented.