In a trench shape measuring apparatus, a substrate having a trench pattern extending in a predetermined trench direction on a measurement area is held by a holding part. A light emission part applies illumination light to the measurement area and reflected light of the illumination light from the measurement area is spectrally dispersed by a diffraction grating of a spectroscope, to acquire a measured spectral reflectance. Since the diffraction grating is arranged so that an angle formed between a direction on the substrate corresponding to a grating direction of the diffraction grating and the trench direction becomes 45 degrees, even if an oscillation direction of the reflected light from the substrate is limited by influence of the trench pattern, it is possible to accurately obtain a spectral reflectance of the measurement area without influence of polarization of the reflected light and obtain a depth of the trench pattern with accuracy.