A device for detecting the mix ratio of a composite liquid (6) has a transparent column (2) placed so that the bottom (22) of the column (2) is in contact with the liquid and a light emitting diode (3) disposed so that light therefrom is incident on one side of the column to pass therethrough so as to be incident on the interface (22a) between the bottom of the column and the mixing liquid. Light incident on the interface at less than the critical angle are refracted, while light is incident on the interface at more than the critical angle is totally reflected to escape from the other side of the column. A photodiode (4) is provided to receive the light beams escaping from the other side of the column and to generate an output. The effective photo-receiving area (41) of the photodiode is in the form of an inverted trapezoid to make the output from the photodiode more linearly dependent on the mix ratio.