In an in-focus state detection device for a camera for forming a plurality of images based on light fluxes transmitted through different areas of a pupil of an imaging lens and detecting the state of the imaging lens based on a relative positional relationship of the images, an optical system for forming the images and a sensor for sensing the images are constructed in a unit through two opposing mirrors to allow ready mounting of the optical system and the sensor, and the positions of the optical system and the sensor are adjusted through the unit member.