A photoelectric conversion device has pixel comprised of a first semiconductor region of a first conductivity type and a second semiconductor region of a second conductivity type disposed in the first semiconductor region. A first diffusion region of the first conductivity type is held at a predetermined potential, covers entirely the first semiconductor region, and covers only a part of an upper portion of the second semiconductor region. A second diffusion region of the second conductivity type covers a part of the upper portion of the second semiconductor region except for the part of the upper portion of the second semiconductor region covered by the first diffusion region. A thick oxide film covers entirely the first diffusion region and covers the upper portion of the second semiconductor region except for the part of the upper portion of the second semiconductor region covered by the second diffusion region.