A Successive Approximation Register (SAR) analog-to-digital converting apparatus includes a reference voltage supply unit suitable for supplying different reference voltages depending on bits of a pixel output signal to be converted, an N bit SAR analog-to-digital conversion unit suitable for sequentially converting upper N−1 bits and lower N bits of the pixel output signal by selectively using the reference voltages supplied from the reference voltage supply unit, where N is a natural number, and an error correction unit suitable for calculating an error correction value based on a difference between conversion results of the lower N bits, and outputting a 2N−2 bit analog-to-digital conversion result by combining converted upper N−1 bits and converted lower N bits and correcting an error of the reference voltages using the error correction value in the combining.