A photoelectric conversion element for detecting the spot size of incident light, including a photoelectric conversion substrate provided with two main surfaces, and multiple first sensitivity sections and second sensitivity sections arranged in a prescribed direction. When sensitivity regions on the respective main surfaces of the multiple first sensitivity sections are defined as first sensitivity regions, and sensitivity regions that appear on the main surfaces of the second sensitivity sections are defined as second sensitivity regions, each of the first sensitivity regions receives at least a part of light incident on the main surfaces, and has a pattern in which, in accordance with enlargement of an irradiation region irradiated with incident light on the main surface, the proportion of the first sensitivity regions in the irradiation region with respect to the first sensitivity regions other than those in the irradiation region and the second sensitivity regions is decreased.