It judges whether a decelerator, such as fuel cut, over-drive cut, down shifting, braking, is in a temporarily unusable condition and further judges that it is necessary to drive the decelerator by the inter-vehicle control. It continues control by the inter-vehicle control means when it is necessary to drive the decelerator by the inter-vehicle control and it is judged that the decelerator is in the temporarily unusable condition. It informs the driver of data in the temporary unusable condition and further inform the driver of alarm data when the actual inter-vehicle distance is shorter than a reference. The data agrees with the alarm data. The same operation may be effected regarding accelerator. The temporarily unusable condition may be caused by continuous operation of brake pressure actuator longer than the rating.