A range finder for measuring the distance of an object to be measured by the steps of radiating the spot light to the object, forming an image of the reflected ray of light by image forming means and measuring the distance of the object from the output of an element which is disposed at a position substantially corresponding to the image forming position and of which output varies in accordance with the light-receiving position on the light-receiving surface. PSD are used as the element. A focussing detector which includes a semiconductor position detection element PSD placed at a position corresponding to the film surface in a camera or the like and detects the focussing state on the basis of the electric output from the semiconductor position detection element PSD. A focussing detector which comprises image forming means for forming the image of an object; a PSD disposed in such an arrangement that the light-receiving surface thereof is eccentric with respect to the optical axis of an optical system including the image forming means and disposed at the back of the image forming means; and an operation circuit detecting the extreme values of the output of the PSD and producing a focussing signal.