A photoelectric conversion device includes a plurality of pixels and a readout circuit. Each of the plurality of pixels includes a pixel circuit including a photoelectric conversion unit. The readout circuit is configured for reading out a signal from the plurality of pixels. The pixel circuit of each of the plurality of pixels includes at least a first transistor that receives a signal based on signal charges generated by the photoelectric conversion unit and is a part of a differential pair. The pixel circuit or the readout circuit has a second transistor. The size of the first transistor and the size of the second transistor are different from each other.