An optical measurement device capable of improving optical spectrum measurement accuracy without the need to structurally decrease a slit width. A diffraction grating for dispersing measurement light into respective different wavelengths is rotated in a given direction to produce diffracted light of selected wavelengths. A focusing lens converges the diffracted light to produce a converged beam. A slit control section varies the slit width at a constant scan speed to open or close the slit, thereby varying the passing bandwidth for the converged beam. A light receiving/measuring section receives the light passed through the slit, obtains a level function indicative of the power level of the received light that varies with change in optical frequency, and differentiates the level function by the scan speed to reproduce the spectrum profile of the measurement light.