An optical sensor includes: a light-emitting unit; a light-receiving unit that receives light radiated from the light-emitting unit and reflected from a detection target and that outputs an output value in response to the light received; and a correcting unit that corrects the output value of the light-receiving unit when receiving the light reflected from the detection target based on the output value of the light-receiving unit obtained by irradiating a detection area of the optical sensor with light without any light reflective objects being present in the detection area.