The present invention provides, as one aspect, an apparatus for detecting an abnormality of a generator for vehicles. The generator includes a multi-phase armature winding that has phase windings and a rectifier that rectifies multi-phase output of the armature winding. The apparatus includes a first period detection unit that detects a first period of an output voltage of the generator, a second period detection unit that detects a second period of an voltage of one of the phase windings, and an abnormality determination unit that determines presence or absence of an abnormality of the generator by comparing the first period with the second period.