A photoelectric conversion device of an embodiment includes a plurality of pixels, a signal processing circuit, and a switch unit. The signal processing circuit performs analog-to-digital conversion on signals output from the plurality of pixels. The switch unit performs switching between a first mode for motion detection to input a first signal generated by adding signals of at least two pixels to the signal processing circuit and a second mode to input respective signals of the at least two pixels to the signal processing circuit individually as second signals. A first period from start to end of a readout operation performed by the signal processing circuit in order to process the first signal is shorter than a second period from start to end of a readout operation performed by the signal processing circuit in order to process one of the second signals.