The invention provides an image sensor and an image capturing apparatus. An image sensor of a photon counting type that suppresses a reduction in the efficiency of photon detection dependent on a pixel position, is disclosed. The image sensor comprising a pixel region in which a plurality of pixels are arrayed in a matrix, each pixel including a photoelectric conversion region and an avalanche breakdown region. In a plan view of a pixel, a size of the avalanche breakdown region is smaller than a size of the photoelectric conversion region. In addition, at least some of pixels arranged in a peripheral region of the pixel region, the avalanche breakdown region is formed such that a position thereof is shifted with respect to a position of the avalanche breakdown region in pixels arranged ina central region of the pixel region.