Provided is an autofocus device (400) comprising: an optical source (1); an optical unit (10;20) that allows emitted light from the optical source and object light from a measurement target (W) to pass through the same optical path, the optical unit being arranged between the optical source and the measurement target; and a detection unit (4A,4B) that performs focus detection by using the object light passed through the optical unit, wherein the optical unit comprises: a tube lens (11;21) that forms the emitted light into parallel light; a line-like image forming unit (12,13;22) that forms a line-like image (L1,L2) on a surface of the measurement target (W) by the parallel light from the tube lens; and a rotation unit (121,122;222) that rotates the line-like image.