An apparatus for detecting an angle of rotation is disclosed which comprises a rotary magnet having four poles, two magnetic sensors of a ferromagnetic material disposed over the periphery of the rotary magnet so as to face the magnetic poles of the rotary magnet and to have a phase difference of 3.pi./8 therebetween, a signal generating portion for applying the magnetic sensors of a ferromagnetic material with a first and a second sine-wave signals having a phase difference of .pi./2 in electrical angle therebetween, a synthesizing portion for synthesizing signals output from both of first magnetic sensor of a ferromagnetic material and second magnetic sensor of a ferromagnetic material thereby to output a synthesized signal with a phase difference corresponding to an angle of rotation .theta. from the first sine-wave signal, and a rotational angle detecting portion for detecting the angle of rotation based on the phase difference.