An autofocus device including: an optical source; an optical unit that allows emitted light from the optical source and object light from a measurement target to pass through the same optical path, the optical unit being arranged between the optical source and the measurement target; and a detection unit that performs focus detection by using the object light passed through the optical unit. The optical unit includes: a tube lens that forms the emitted light into parallel light; an image forming unit that forms a continuous line image on a surface of the measurement target; and a rotation unit that rotates the continuous line.