PURPOSE: To prevent an error in converted digital angle data even if an analog rotational angle signal voltage which is inputted all the time varies by performing S/D conversion while detecting the maximum value of the amplitude of the analog rotational angle signal voltage.

CONSTITUTION: A signal necessary for S/D conversion is e01 and expressed by an equation I . A comparison angle is the contents of a Up/Down counter and controlled by the counter. When θ=, e01=0, and the contents of the counter are exactly digitized θ. The absolute value vector of a carrier, i.e. maximum value is detected and amplification factors A3 and A4 of a signal system are controlled on the basis of the detected value to correct the absolute value of the maximum amplitude of the carrier into a constant value. A signal necessary for operation is e03 and expressed by an equation II. When =θ, the level of the carrier is detected and corrected so that is coincides with a reference value, thereby correcting the maximum amplitude of the carrier. Consequently, even if the analog rotational angle signal voltage varies, the converted digital angle data has no error.