To attain a trench depth measuring apparatus for measuring the depth of a trench with high aspect ratio, whose trench width is at the same degree as wavelength of illumination light.

An illumination optical system has: light source devices 1-4 which generate line-like illumination beams; and an objective lens 6 which projects the line-like illumination beams toward a sample to form a line-like pupil pattern in parallel with the longitudinal direction of a trench at a pupil position between the light source devices and the objective lens. The line-like pupil pattern forms a line-like illumination area so as to intersect the trench on the sample surface 7 where the trench is formed via the objective lens. In addition, linearly polarized illumination light is used as the illumination light, and a direction of its electric field vector is set approximately in parallel with the longitudinal direction of the trench. By setting the direction of the electric field vector of the linearly polarized illumination light in the longitudinal direction of the trench, optical loss in the trench decreases, the illumination light can be entered inside the trench, and highly accurate depth measurement becomes possible.