Abstract: To provide a fuel property judgement device for an internal combustion engine, in which, without increasing costs, fuel property is judged earlier and an optimum control suitable for the fuel property is carried out earlier whereby the predicted degradation of the controllability at the time of starting operation, caused by the difference in fuel property, immediately after the start and during a transient period may be prevented, there are provided a unit for detecting an initial combustion at the time of starting operation of the internal combustion engine, a unit for counting a turn-on number of a crank angle signal until the initial combustion is detected, a unit for setting a standard number, a unit for comparing the turn-on number with the standard number, and a unit for determining the fuel property on the basis of a comparison result.