An image sensor, comprising: a plurality of pixels, each of the pixels comprising a photoelectric conversion unit that photoelectrically converts incident light to generate an electric charge; and an AD conversion unit; wherein: the AD conversion unit comprises: a comparison unit that compares a signal caused by an electric charge generated by the photoelectric conversion unit with a ramp signal varying with time; a first storage unit provided in a first circuit layer, the first storage unit storing a first signal based on a result of a measurement with a clock signal having a first frequency, the result representing a time until a magnitude relationship changes between the signal caused by the electric charge generated by the photoelectric conversion unit and the ramp signal, when the comparison unit compares the signal caused by the electric charge generated by the photoelectric conversion unit with the ramp signal; and a second storage unit provided in a second circuit layer that is stacked on the first circuit layer, the second storage unit storing a second signal based on a result of a measurement with a clock signal having a second frequency that is lower than the first frequency, the result representing a time until a magnitude relationship changes between the signal caused by the electric charge generated by the photoelectric conversion unit and the ramp signal, when the comparison unit compares the signal caused by the electric charge generated by the photoelectric conversion unit with the ramp signal, wherein a distance between the comparison unit and the first storage unit is shorter than a distance between the comparison unit and the second storage unit. a plurality of pixels, each of the pixels comprising a photoelectric conversion unit that photoelectrically converts incident light to generate an electric charge; and an AD conversion unit; wherein: the AD conversion unit comprises: a comparison unit that compares a signal caused by an electric charge generated by the photoelectric conversion unit with a ramp signal varying with time; a first storage unit provided in a first circuit layer, the first storage unit storing a first signal based on a result of a measurement with a clock signal having a first frequency, the result representing a time until a magnitude relationship changes between the signal caused by the electric charge generated by the photoelectric conversion unit and the ramp signal, when the comparison unit compares the signal caused by the electric charge generated by the photoelectric conversion unit with the ramp signal; and a second storage unit provided in a second circuit layer that is stacked on the first circuit layer, the second storage unit storing a second signal based on a result of a measurement with a clock signal having a second frequency that is lower than the first frequency, the result representing a time until a magnitude relationship changes between the signal caused by the electric charge generated by the photoelectric conversion unit and the ramp signal, when the comparison unit compares the signal caused by the electric charge generated by the photoelectric conversion unit with the ramp signal, a comparison unit that compares a signal caused by an electric charge generated by the photoelectric conversion unit with a ramp signal varying with time; a first storage unit provided in a first circuit layer, the first storage unit storing a first signal based on a result of a measurement with a clock signal having a first frequency, the result representing a time until a magnitude relationship changes between the signal caused by the electric charge generated by the photoelectric conversion unit and the ramp signal, when the comparison unit compares the signal caused by the electric charge generated by the photoelectric conversion unit with the ramp signal; and a second storage unit provided in a second circuit layer that is stacked on the first circuit layer, the second storage unit storing a second signal based on a result of a measurement with a clock signal having a second frequency that is lower than the first frequency, the result representing a time until a magnitude relationship changes between the signal caused by the electric charge generated by the photoelectric conversion unit and the ramp signal, when the comparison unit compares the signal caused by the electric charge generated by the photoelectric conversion unit with the ramp signal,