An accuracy error for measuring the position of an own vehicle is calculated based on the information of an execution history of map matching and the information of an update history of a map database. In addition, a predetermined map is referred to based on the calculated accuracy error, and an accuracy error level for performing positioning is specified from among plural stepwise levels. Then, the control level of assist control executed according to the position of the own vehicle is changed so as to correspond to the specified accuracy error level.