An imaging device in which signals can be read out accurately at high speed is provided. The imaging device includes a plurality of pixels arranged in a matrix, an A/D converter circuit, and a selector circuit. The pixels are electrically connected to an input terminal of the A/D converter circuit. An output terminal of the A/D converter circuit is electrically connected to one of a source and a drain of a transistor. The other of the source and the drain of the transistor is electrically connected to an input terminal of the selector circuit. The transistor includes an oxide semiconductor in an active layer. Other embodiments are described and claimed.