A thickness monitoring device capable of performing thickness measurement of an object of measurement in real time even when using a relatively narrow band light source. This etching monitoring device (thickness monitoring device) includes a light source which produces measurement light having a predetermined wavelength bandwidth; an array detector which detects, for each wavelength, interference light of measurement light reflected from mask, whereof the thickness changes over time; and data processing unit which computes the thickness of the mask based on change over time of a plurality of single-wavelength components of the interference light detected by the array detector.