PURPOSE: To easily measure mixing ratio by stirring and mixing a prescribed amount of alcohol mixed gasoline (gasahol) and water then leaving the mixing to stand still and detecting the difference between the interface of the gasoline and water and the interface prior to stirring.

CONSTITUTION: For example, 50ml water is taken into a measuring cylinder, and 50ml gasahol is gently put thereon. Next, after the mixture is thoroughly stirred, it is allowed to stand still, then the alcohol in the gasahol dissolves in the water layer, and the position of the interface with the gasoline and water changes. If the interface positiong by a known alcohol mixing ratio is beforehand measured, the mixing ratio of the gasahol to be measured is easily determined.