PURPOSE: To prepare gasohol which does not separate into two layers to separate water at a low temp., by decreasing a proportion of associated alcohol in a raw alcohol/gasoline mixt. by subjecting the mixt. to supersonic treatment and bringing the cavitation effect into action.

CONSTITUTION: Raw alcohol and gasoline are allowed to flow via passages 12, 13 and 11 into a hollow part 10 of a treater 9 having an opened head and a supersonic generator 2 is turned on to vibrate an oscillator 5 connected with the generator 2. The vibration is transferred via a horn 8 inserted into the hollow part 10 of the treater 9 to a mixt. of the alcohol and gasoline, and the associated ethyl alcohol molecule and water molecule in the raw alcohol are dissociated by the cavitation effect caused by vibration energy of the supersonic wave and dispersed in the gasoline in a state of a single molecule. After completely mixed, the product is withdrawn via a discharge passage 14.