A device has stacked first and second substrates. The first substrate includes pixel array having pixels arranged along surface, the second substrate includes readout circuit having first AD-conversion circuit for generating first digital signals corresponding to first signals from the pixels, first parallel-serial conversion circuit for outputting first serial signal by performing parallel-serial conversion on the first digital signals, and processing circuit for processing the first serial signal. In orthogonal projection to the surface, part of the pixel array and part of the processing circuit overlap with each other. The first AD-conversion circuit is arranged between edge of the second substrate and the processing circuit. The first parallel-serial conversion circuit is arranged between the first AD-conversion circuit and the processing circuit.