To prevent erroneous judgement by a method wherein a fuel nature not influenced by an amount of fuel adhered to a wall surface before reaching a transient time can be detected.

As a fuel cut condition is established and a fuel cut is completed (S220), it is judged whether or not an accelerated state is attained from a completion of fuel cut within a predetermined time 10 (S260). In this case, if it is judged that an accelerated state is established, a varying amount AF of the detected value of the air fuel ratio sensor is calculated (S270 to S300) and then a heavy quality fuel is judged in reference to the varying amount AF (S310 to S320). Upon shut-off of supplying fuel due to cut of fuel, fuel adhered to the intake air passage or the wall surface of the cylinder is sucked into the combustion chamber due to a negative pressure of the internal combustion engine, resulting in that an amount of fuel adhered to the wall surface is uniformly reduced and so the acceleration is carried out from a state showing a uniform less amount of adhesion of fuel at the wall surface so as to cause a judgement of the nature of the fuel to be carried out and then an erroneous judgement can be prevented.