A motion detection method includes an extracting circuit generating a first representative data according to a first image in an image signal outputted by an image sensor, a buffering circuit storing the first representative data, the extracting circuit generating a second representative data according to a second image in the image signal, the buffering circuit storing the second representative data and outputting the first representative data to a comparing circuit, and the comparing circuit comparing the first representative data and the second representative data, to generate a motion detection result. When the motion detection result represents “motion”, an image-signal-processing circuit of an image signal processor enters active state to generate output signal. By means of the motion detection method, in an imaging device, the image-signal-processing circuit of the image signal processor can switch between active state and power-off/power-saving state even when there is no additional infrared sensor.