containing the principal rays l3 and l4 travelling towards the field mask fm so as to pass away from the optical axis of the objective lens at a predetermined angle relative to the optical axis passes through the respective rectangular opening e01 in the field mask fm and then through the respective condenser lens l01 the off axial light flux having passed through the condenser lens l01 is deflected by the condenser lens l01 so as to travel towards the optical axis and then projected onto the associated ccd line sensor p01 through the paired apertures a3 and a4 in the aperture mask am the paired image re forming lens l3 and l4 in the image re forming lens plate l and the aperture no1 in the shield member nm in this way paired images are re formed on the ccd line sensors p01 on the other hand the other light flux for the other off axial focus detection area that is the other off axial light flux containing the principal ray l5 and l6 travelling towards the field mask fm so as to pass away from the optical axis at the predetermined angle relative to the optical axis passes through the respective rectangular opening e02 in the field mask fm and then through the respective condenser lens l02 and are subsequently similarly projected onto the associated ccd line sensor p02 through the paired apertures a5 and a6 in the aperture mask am the paired image re forming lens l5 and l6 in the image re forming lens plate l and the aperture no2 in the shield member nm in this way paired images are re formed on the ccd line sensors p02 the light flux.