PURPOSE: To improve the measuring accuracy by combining a rod-shaped ray and a light receiving element unit with a transparent measuring pipe.

CONSTITUTION: A rod-shaped light source 3 such as a fluorescent lamp is arranged in parallel with and substantially over the whole length of the measuring pipe 1 in the vicinity of the output wall of the same. The measuring pipe 1 is made transparent and has the same internal diameter and height as those of a measuring cylinder of one litter. The measuring pipe 1 is formed with a scale 2 indicating the liquid level when one litter of the liquid to be measure is poured. A plurality of light receiving elements 6 are arranged in a row equidistantly in the minimum pitch for the size thereby to constitute light receiving element units 4 and 5. The center of the upperpost element 6 of the unit 4 is aligned with the scale line 2, and the unit 5 is shifted down one half pitch with respect to the unit 4. Thus, the units 4 and 5 constitute an optical scale 7, which has the detecting accuracy twice that for the single unit so that the position of the boundary 10 between the supernatant liquid 8 and the sedimented layer 9 can be highly accurately detected.