An object of the present invention is to provide a photosensor unit with a structure which is simple and does not lower the detection accuracy. Provided is a photosensor unit configured such that reflected light of detection light emitted from one or more light emitting elements 2 housed in a unit case 1 to the outside of the unit case 1 is detected with a light receiving element 4 mounted on a mounting board 3, the reflected light being reflected from a detection region set on the outside of the unit case 1. A passage space for the reflected light inside the unit case 1 is separated from the other space in the unit case 1 by a partition member 6 placed on the mounting board 3 with an annular packing 5 therebetween which surrounds the light receiving element 4 and is made of an elastic material. The partition member 6 is provided with a pressing wall 7 configured to come into pressure contact with the outer peripheral wall of the annular packing 5 when the partition member 6 is placed on the mounting board 3, and thereby apply pressure contact force to the annular packing 5 while dividing the pressure contact force in a radially compressing direction, the pressure contact force being force with which the annular packing 5 comes into contact with the mounting board 3.