A rotation-angle-detecting apparatus comprising a magnet rotor, a magnetic sensor detecting the direction of magnetic flux from the magnet rotor, a correction circuit, and an angle-calculating circuit, the magnetic sensor having bridge circuits X and Y each comprising four connected magnetoresistive devices, each magnetoresistive device comprising a spin-valve, giant-magnetoresistive film, the correction circuit calculating difference (Vx−Vy) and sum (Vx+Vy) from the output voltage Vx of the bridge circuit X and the output voltage Vy of the bridge circuit Y, and making their amplitudes equal to each other, and the angle-calculating circuit determining the rotation angle of the rotor by arctangent calculation from a signal (Vx−Vy)′ and a signal (Vx+Vy)′ supplied with the same amplitude from the correction circuit.