of the ultrasonic wave the microcomputer 70 checks at step 270 whether the ultrasonic vibrator 10 has an accumulation of snow thereon the microcomputer 70 determines accumulation of snow if the calculated period of reverberating vibration is more than 70 of a predetermined time period t 6 2 5 ms which is after 1 2 ms from the start of ultrasonic wave transmission if no snow accumulates on the ultrasonic vibrator 10 the reverberating vibration continues for the period t 4 as shown in fig3 however if snow accumulates on the ultrasonic vibrator 10 the reverberating vibration continues for the period t 5 as shown in fig4 the period t 5 is longer than the period t 4 it was confirmed in the experiments that accumulation of snow can be detected by checking whether the time period t 4 or t 5 is in excess of a predetermined ratio 70 of the predetermined time period t 6 under the condition that the detected temperature is 0 u00b0 c if the check result at step 270 is no indicating no snow accumulation the microcomputer 70 executes steps 230 250 if the check result is yes indicating snow accumulation the microcomputer 70 causes at step 271 the driver circuit 90 to drive the display 80 so that abnormality of the ultrasonic vibrator 10 may be displayed with this display a driver of the vehicle is enabled to recognize that.