Image sensor and control method therefor, and image capturing apparatus

An image sensor comprising: a generator that generates a plurality of reference signals having different slopes of potential change; a selector that selects one of the reference signals; and an analog/digital converter that converts an analog signal output from a pixel unit by first or second driving using the selected reference signal into a digital signal. In the first driving, a noise signal is converted into a plurality of first digital signals using the reference signals and a photoelectric conversion signal is converted into a second digital signal using one of the reference signals. In the second driving, the noise signal is converted into the first digital signal using a predetermined one of the reference signals and the photoelectric conversion signal is converted into the second digital signal using one of the reference signals. The first driving is performed intermittently.