A concentration detector is adapted to detect the concentration of a liquid fuel in a container. The concentration detector comprises a rotating mechanism positioned underneath the level of the liquid fuel and having a rotational center. A first float having a specific gravity less than the specific gravity of the liquid fuel, a volume equivalent to "V" and a mass equivalent to "M". The distance between the mass center of the first float and the rotational center is "L". A second float connected with the first float and having a specific gravity less than the specific gravity of the liquid fuel, a volume equivalent to "V" and a mass equivalent to "r*M". The distance between the mass center of the second float and the rotational center is "L. The mass center of the second float, the rotational center and the mass center of the first float constitute an included angle of 120.degree.. A third float connected to the first float and the second float and having a specific gravity less than the specific gravity of the liquid fuel, a volume equivalent to "V" and a mass equivalent to "r.sup.2*M". The distance between the mass center of the third float and the rotational center is "L. The mass center of the third float, the rotational center and the mass center of the second float constitute an included angle of 120.degree.. Thereby the variation in the concentration of the liquid fuel is acquired by inspecting the change in the rotational direction of the rotating mechanism.
