A polyphase AC generator of a type designed for installation in automobiles, has a so-called Lundell type rotor in which an exciting coil is enclosed by a plurality of pairs of finger type magnetic poles and which is rotated by a vehicle-mounted engine. The shape of the finger magnetic poles is modified to an irregular trapezoidal shape having a side opposite to a base displaced with respect to said base in the direction of rotation of the rotor, thus changing the waveform of magnetic flux flowing through the rotor and an armature core and thereby reducing magnetic noise of the generator under load operation.