Abstract: In the disclosed automatic focus detecting device for an imaging optical system a light spot is projected onto an object and an image of the object is formed on a predetermined focal plane. A light receiving photo-sensitive element produces at least two signals according to the position of the reflection of the projected light spot. A comparator detects when the relation between the integrated values of outputs of the light receiving element exceeds a predetermined level. A detector means detects when the light spot image has been projected for a predetermined time and a determining arrangement determines the focus of the imaging optical system on the basis of the outputs of the light receiving element when at least either the comparator or time detector detects the predetermined level or length of time.