PURPOSE: To enable the detection of focusing state with high accuracy by deviating the respective arrangements of lenses corresponding to two image-forming luminous fluxes from each other roughly by a half pitch and disposing the same in proximity in parallel with each other.

CONSTITUTION: If, for example, an out-of-focus image 26 of a narrow slit width having a negligible width is projected on lenslet arrays as shown in the figure, the luminous flux is made incident only to the one lenslet with one row of photodetectors in the prior art and is therefore judged to be focusing and in the worst case, the out of-focus is not detected unless the width (d) of the out-of- focus image is made equal to the pitch P of the lenslet. If the two photodetectors are used in contrast with the prior art, the luminous flux is judged to be focusing on the lenslet array 25-1 but the luminous flux is made incident to the two lenslets 27-1, 27-2; therefore, the out-of-focus is detected.