A photoelectric conversion element for detecting the spot size of incident light. The photoelectric conversion element includes a photoelectric conversion substrate having two principal surfaces, and the substrate includes a first sensitivity part and a second sensitivity part that are separated from each other. When a sensitivity area appearing on the principal surface of the first sensitivity part is defined as a first sensitivity area and a sensitivity area appearing on the principal surface of the second sensitivity part is defined as a second sensitivity area, the first sensitivity area receives at least a portion of incident light incident on a light receiving surface, and a pattern is formed such that an increase in an irradiation area of the principal surface irradiated with the incident light reduces the ratio of the first sensitivity area to the second sensitivity area in the irradiation area.