Abstract: A detecting float is normally made floating on a first liquid, and when a liquid to be detected having a specific gravity less than the first liquid is oncoming, the detecting float sinks relative to the new liquid level as compared with the position which it assumed relative to the level of the first liquid. When the layer of the liquid to be detected increases to a given thickness, the detecting float sinks by a given distance or amount to permit the ingress of the liquid to be detected into the float, which therefore becomes heavier and sinks further downward. An external float surrounds the detecting float, and is formed with a liquid port at a location slightly below the level of the first liquid for allowing the passage of the liquid which reaches the internally located detecting float, but intercepting the liquid to be detected which is in the form of an oil film.