<p>An encoder includes a scale member provided on one body of two bodies which are relatively rotated and having gratings arranged in a relative rotating direction and a light irradiation means for irradiating a first region with beams. A diffracted image of the first regions is projected on a second region of the scale member. The encoder further includes a means for detecting the beams from the second region. The relative rotation of the other body is detected based on a detected result thereof. Disclosed is a scale whose relative rotation to the rotation detecting device is detected by its being irradiated with the beams from this device. The scale includes a base member formed with the grating having surfaces inclined to incident beams. Disclosed is a driving system for causing relative rotations of the two bodies. This system includes a means for relatively rotationally driving the two bodies and a control means for controlling a driving condition of the driving means. Disclosed is a motor unit including a motor module having its body whose rotation is detected by a result of a light irradiation means and rotary shaft provided with a scale member. This unit further includes a means for integrally supporting the body, the light irradiation means and the detecting means. <IMAGE></p>
