A navigation apparatus carries out a diagnosing process for vehicle behavior, for example, whether sudden accelerating operation or sudden braking operation has been done, wherein the diagnosing process is carried out based on vehicle acceleration. It is necessary to accurately calculate the vehicle acceleration in order to perform a reliable diagnosing process. The navigation apparatus calculates a difference between a first vehicle acceleration defined by an output from an acceleration sensor and a second vehicle acceleration calculated based on a vehicle speed and vehicle angular speed. The navigation apparatus carries out the diagnosing process, for example, when the above difference between the first and second vehicle accelerations is less than a predetermined value.