Photoelectric conversion device

A photoelectric conversion device for detecting the spot size of incident light, includes a photoelectric conversion element having a photoelectric conversion substrate with two main surfaces, and first and second sensitivity section sections; and scanners that relatively scan incident light on the main surfaces of the photoelectric conversion element. When a sensitivity region on a main surface of the first sensitivity section is defined as a first sensitivity region and sensitivity regions that appear on a main surface of the second sensitivity sections are defined as second sensitivity regions, the first sensitivity region receives at least part of the light incident on the main surface during scanning, 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 region with respect to the second sensitivity regions in the irradiation region is decreased.