The invention provides a device which utilizes the tunnel effect occuring upon a condition of geometric total reflection, for measuring a narrow gap and surface unevenness of a specimen with high precision. An optical device 40 includes a semi-conductor laser 42, a photodiode 43, and a waveguide layer 44 is formed on a semi-conductor substrate 41 by epitaxial growth. A reflecting surface 44b of the waveguide layer 44 is parallel to the plane of the semi-conductor substrate. A laser beam emitted from the semi-conductor laser is reflected from the reflecting surface 44b under a condition of total reflection in geometrical optics. When the gap between the reflecting surface and the specimen is less than or equal to about the wavelength of the laser beam, part of the laser beam is transmitted into the specimen. The intensity of the transmitted light, which is calculated corresponding to the intensity of the reflected light, depends on the dimension of the gap. In the practice of the invention, the relationship between the transmittance and the dimension of the gap is previously obtained, and the dimension of the gap is determined corresponding to the transmittance measured. A narrow gap and the surface unevenness are accurately and precisely measured.