Abstract: A synchro-to-digital converter converts polyphase a.c. signals, delivered from a synchro, a resolver, or the like (hereinafter called simply a synchro) in correspondence to a rotational angle of the rotor thereof, into digital values. Two phase signals, adjacent to each other in the polyphase synchro signals, are selected in a predetermined mode by the use of higher bit signals obtained from an operational device, and are thereafter introduced into a D-A (digital-analog) converter driven by lower bit signals obtained from the operational device. The output signal from the D-A converter is compared with a reference signal so that the polarity thereof is discriminated. The operational device is continuously driven by the polarity-discriminated until the output signal from the D-A converter becomes zero. Thereby absolute digital values corresponding to the rotational angle of the rotor can be delivered at a high speed.