To convert link travel times into vehicle speeds and then correct converted vehicle speeds judged to be anomalous. A link travel time TT in each link is converted into a vehicle speed V at every time.On the basis of a maximum variation [Delta]Vmax; a speed variation [Delta]V1between the vehicle speed V and a vehicle speed Vb previous by a unit time; and a speed variation [Delta]V2between a vehicle speed Va later by the unit time and the vehicle speed V, the vehicle speed is corrected by adding the maximum variation [Delta]Vmax to the vehicle speed Vb if the code of the speedvariation [Delta]V1is not the same as the code of the speed variation [Delta]V2. If the code of the speed variation [Delta]V1is the same as the code of the speed variation[Delta]V2, the vehicle speed is corrected in such a way as to take the same value as a means value of the vehicle speed Vb previous by the unit time and the vehicle speed Va later by the unittime.