A photosensor includes a light projection unit, a light receiving unit, a determination unit that determines a magnitude relation between a received light quantity of the light receiving unit and a threshold value set for the received light quantity; and a threshold value setting unit that sets the threshold value of the received light quantity. The threshold value setting unit calculates for each of a plurality of different wavelength regions, a contrast difference that is a difference between a received light quantity obtained when projection is performed on a detection target on a surface of a workpiece and a received light quantity obtained when projection is performed on a background on the surface of the workpiece, and updates the threshold value based on the contrast difference for a wavelength region among the plurality of wavelength regions in which the contrast difference has a maximum value.