An imaging device with low power consumption is provided. It includes a pixel capable of outputting difference data between two different frames, a circuit determining the significance of the difference data, a circuit controlling power supply, an A/D converter, and the like; obtains image data and then obtains difference data; and shuts off power supply to the A/D converter and the like in the case where it is determined that there is no difference, and continues or restarts the power supply to the A/D converter and the like when it is determined that there is a difference. Determining the significance of the difference data can be performed row by row in a pixel array or at nearly the same time in all the pixels included in the pixel array.