Rotational angle sensor, motor, rotational angle detector, and electric power steering system

A rotational angle sensor includes: a magnet rotor in which multiple magnetic poles are formed along the circumferential direction of the magnet rotor; and three sensor devices (33) that are arranged on a circle concentric with the magnet rotor at regular angle intervals. Each of the sensor devices (33) includes three half-bridge circuits (38) each of which has a pair of spin valve magnetic resistances (37a, 37b) that are connected in series in such a manner that the magnetization directions of spin fixed layers of the spin valve magnetic resistances (37a, 37b) are opposite to each other. Each of the sensor devices (33) is formed in such a manner that phases of the sensor signals (Vu, Vv, Vw) output from the respective half- bridge circuits (38) based on a change in magnetic flux caused by the rotation of the magnet rotor are offset from each other by an electrical angle of 120 DEG .