To determine a degree of dirt on a translucent plate from a received light waveform used for distance measurement so as to output an alarm signal by determining a very close distance computed by means of a distance computing means as a computed distance based on dirt on the translucent plate and canceling this error computed distance.

It is determined whether a computed distance to the front object is not more than 1m or not (ST108). When the computed distance is 1m or less, it is determined that its peak waveform is very likely to indicate dirt. Then, it is determined whether a peak value of the peak waveform indicating the dirt exceeds 0.7 times the value of the maximum wave height value or not (ST109). If the peak value does not exceed 0.7 times the value of the maximum wave height value and it is determined it is dirt, a dirt alarm signal is fed to an alarm circuit (ST110). If the peak value exceeds 0.7 times the value of the maximum wave height value and it is determined that it is not dirt, the signal is determined as a distance signal.