Abstract: The deterioration of receiving environment of a reflected signal from a preceding vehicle in an auto-cruise control is immediately detected, so that the distance between the vehicle and the preceding vehicle detected by a distance sensor comprising a laser radar, is maintained constant. On the basis of data of the magnitude of received signals, which are detected by the distance sensor and accumulated over a predetermined time, a standard deviation (or a dispersion) and an average value of the magnitude of the received signal are calculated. If the standard deviation is equal to or larger than a reference value or the average value is equal to or smaller than a reference value when the distance sensor has not detected a reflected signal, it is determined that the environment of reception of the reflected signal from the preceding vehicle has deteriorated, whereby the auto-cruise control is discontinued, or the deterioration of the receiving environment is informed to a driver by an informing device.