Abstract: An optical measuring device for measuring the fat contents of milk comprises a light source, an optical condenser, a glass test tube provided for holding a set amount of the milk the fat contents of which must be measured, a lens, a photosensitive element, for example of the CCD type, and a digital display, all enclosed in a metal housing which also holds a control electric and electronic system, the fat contents of the milk being measured by detecting, by means of the photosensitive element, the diffusion depth of the light projected by the light source on the milk held in the test tube and reflected on the photosensitive element by the fat particles.