PURPOSE: To improve judging accuracy of fuel volatility by detecting variation of factors in relation to a combustion result in a rapid transient condition so as to perform judgment of fuel volatility.

CONSTITUTION: Engine rotational speed is detected by a crank angle from a crank angle sensor 22. It is detected whether heavy gasoline is used or not by a control unit 20 based on the condition of a drop in the speed just after starting an engine 1. In the case when the use of heavy gasoline is not confirmed, it is judged whether the heavy gasoline is used or not accurately by judging variating frequency of the rotational speed at the time of rapid acceleration. Consequently, since generation of variation in wall face adhesive quantity of fuel is utilized, judgment of the fuel volatility can be accurately performed.