The present invention provides an analog-to-digital converter, which comprises an integration circuit, a threshold signal generating circuit, a main comparison circuit, a sub comparison circuit, a counter, and a decoder. The integration circuit integrates an input signal and produces an integration signal. The threshold signal generating circuit generates a main threshold signal and a plurality of sub threshold signals. The main comparison circuit produces a plurality of main comparison signals according the integration signal and the main threshold signal. The sub comparison circuit produces a plurality of sub comparison signals according to the integration signal and the plurality of sub threshold signals. The counter counts the plurality of main comparison signals and produces a first counting signal. The decoder decodes the plurality of sub comparison signals and produces a second count signal. The first count signal and the second count signal are used for producing a digital signal.