The present invention relates to a motion detecting device for detecting the relative motion rate or velocity between an object and an optical instrument in an exposure control system by which the exposure time, i.e. the shutter speed of the instrument is controlled in response to the relative motion rate or velocity. The motion detecting device is designed to detect the displacement ratio or speed of the position of the center of gravity of the light distribution of the object image on the imaging plane, whereby the danger for misdetection due to the periodical fluctuation of the object brightness can be profitably avoided, being possible to detect only the relative movement between the object and the instrument.