To surely align a semiconductor wafer regardless of color of a colored photosensitive resist applied to the wafer, by widening the wavelength band of a light source of an optical unit of a lighting source which lights up the wafer.

An optical unit 21 which lights up the surface of a semiconductor wafer is provided with a light source, for example, a halogen lamp 21a having a wide wavelength band. The wavelength band of the light emitted from the lamp 21a is adjusted to an appropriate band, by successively passing the light through a diffusing plate 21b, a long-wavelength cut filter 21c, and a short-wavelength cut filter 21d. Since the light emitted from the unit 21 can be passed through a colored photosensitive resist applied to the surface of the semiconductor wafer regardless of the color of the resist by adjusting the wavelength band of the light in accordance with the color of the resist, the semiconductor wafer can be aligned by surely detecting alignment marks on the wafer.