An analog-to-digital converting circuit includes: an analog-to-digital converter suitable for performing an analog-to-digital conversion on pixel signals of a plurality of pixels provided in a pixel array; a ramp signal generator suitable for providing a ramp signal to the analog-to-digital converter; and an auto-zero controller suitable for providing a reference voltage to the analog-to-digital converter to perform an auto-zeroing operation by using a row pixel for which a readout operation is performed by the analog-to-digital converter.