According to one embodiment, a digital converter is provided to process output signals of a resolver attached to a rotating apparatus. These output signals are converted into digital signals by ΔΣ AD converters. Multiplication units receive the digital signals outputted from the ΔΣ AD converters. A subtraction unit subtracts the outputs of the multiplication units. A synchronous detection unit detects the output of the subtraction unit based on a synchronization signal synchronously. An angular velocity calculation unit adjusts a proportional gain of an output of the synchronous detection unit, compensates the phase thereof, and calculates the angular velocity of the rotating apparatus. A rotational angle calculation unit calculates a rotational angle by integrating the angular velocity. Cosine and sine output units output a cosine and a sine corresponding to the obtained rotational angle to the multiplication units, respectively.