A focus detecting device which is provided with a field lens disposed on the predetermined imaging plane of an objective lens and a plurality of imaging lenses disposed rearwardly of the field lens and in which the images of the imaging lenses are sensed to thereby determine the focus adjusted state of the objective lens. A unit body integrally constitutes the units such as the field lens and the imaging lenses and has a surface for movably supporting the field lens. The unit body also has adjusting device for rotating the imaging lenses relative to the optical axis.