Abstract: An improved angle measuring device in which rotational irregularity in the form of two cycles or more per rotational cycle of the motor driving device is compensated for completely. A period of time T.sub.1 required for a cutout formed in the periphery of a rotor rotating at a constant speed to make one revolution starting from a stationary reference point, and a period of time T.sub.2 required for the cutout to move between the reference point and an angle measuring point are obtained to evaluate 360.degree..times.(T.sub.2 /T.sub.1) to obtain the angle under measurement. The cutout has an angular width of approximately 90.degree., although angular widths in a range of 5.degree. to 355.degree. can be used as the situation demands.