A surface inspection apparatus is provided based on an optical interference scheme using a wide-band laser light source, such as diode laser, for an interferometer. In the apparatus, a diode laser with a large spectrum width having a short coherence length is used as an emitted light source; modulation optical elements for performing modulation with slightly different frequencies, and optical path length varying optical elements for adjusting the optical path length are located in each of two optical paths between a branching optical element and a combining optical element; and the above-mentioned optical path length varying optical elements are adjusted, while measuring an interference intensity, so as to maximize the interference intensity.