A device for measuring thickness of an object has a vibration generator for generating vibrations in the object, a vibration detector for detecting vibrations generated in the object by the vibration generator and a frequency analyzer for calculating resonance frequency of the object. The vibration generator includes a light-emitting part which emits light towards the object to irradiate and to be absorbed by the object. A plurality of vibration detectors may be used and the frequency analyzer may include a sound speed analyzer for calculating speed of sound inside the object from vibrations detected by these plurality of vibration detectors.