PURPOSE: To detect the presence of a transparent substance by constituting the optical sensor of a cell wherein at least one of light transmitting surfaces is a curved surface, a light emitting surface and a light detecting surface.

CONSTITUTION: A light emitting part 1 using an infrared ray emitting diode and a light detecting part 3 using a PIN infrared photodiode are arranged in opposed relationship and a cell 2 using a glass tube is arranged between them. When light is transmitted through the cell, since the refractive index of the cell 2 is largely different from that of air in such a case that air is present in the cell 2, light is scattered by the curved surface part of the cell 2 and the quantity of the light reaching the light detecting part 3 becomes little. In such a case that a substance to be detected (mainly a transparent liquid) is present in the cell 2, since the refractive index of the cell 2 is not different too much from that of the liquid, the scattering of light by the curved part is not too much and the quantity of the light reaching the light detecting part 3 becomes much. By detecting the difference between both of them, it can be detected whether the substance is present in the cell 2. Further, the substance to be detected can be specified from the quantity of the light reaching the light detecting part 3.