An auto-focus device includes: a light projection lens unit that projects light emitted from a light source unit, on a projection target surface; a light receiving unit that receives a diffusely-reflected light from the projection target surface; a light receiving lens unit that guides the diffusely-reflected light to the light receiving unit; a calculating unit that calculates a focal distance to the projection target surface from the diffusely-reflected light; a first holding unit that holds the light source unit, the light projection lens unit, and the light receiving lens unit; a second holding unit that holds the light receiving unit, and is mounted on the first holding unit; and a pressing unit that presses the second holding unit against the first holding unit. The second holding unit is configured to be displaceable on a surface of the first holding unit along a groove portion formed thereon.