In an analog-to-digital conversion method for converting a difference signal component representing a difference between a reference component and a signal component in an analog signal to be processed into digital data, in a first process, a signal corresponding to one of the reference component and the signal component is compared with a reference signal for conversion into the digital data. Concurrently with the comparison, counting is performed in one of a down-count mode and an up-count mode, and a count value at a time of completion of the comparison is held. In a second process, a signal corresponding to the other one of the reference component and the signal component is compared with the reference signal. Concurrently with the comparison, counting is performed in the other one of the down-count mode and the up-count mode, and a count value at a time of completion of the comparison is held.