A non-invasive device for measuring blood glucose levels comprises an enclosure with a cover and dual openings, wherein the enclosure is covered with a transparent material (e.g., glass or plastic); at least two test cards arranged at specified distances and positioned within the enclosure; an illuminator and a conical light reflecting surface for casting a beam on the test cards. Light from one test card is reflected through one of the dual openings to sensing receptors of one eye. In order to enable the part of a diffuse beam reflected from the second test card made parallel within 10.degree. to fall on the sensing receptors of the second eye, the enclosure is provided with a tubular prism covered with a closing plate having an opening at a first end while having an opening covered preferably with transparent material at a second end.