PURPOSE: To prevent the occurrence of mutual interference of a measuring device and the occurrence of an erroneous measurement, by a method wherein starting times of measuring times of doppler frequencies are staggered from each other by a given time by each electroacoustic converter.

CONSTITUTION: Ultrasonic transmitting receiving heads 11 ans 12 for measuring an ultrasonic doppler frequency corresponding to a speed of a vehicle running on each line of 2 lines or the like are switched so that a transmitting and a receiving wave alternate in the proximity of a point of time when ultrasonic waves running to and from sensing regions 13 and 14 through a timing generating circuit performing a periodical motion. The constitution is made such that their respective stating times of measuring times of doppler frequencies for the heads 11 and 12 are staggered from each other by a given time, and this prevents the occurrence of mutual interference resulting from the receipt of an ultrasonic doppler frequency signal from other head, and the occurrence of an erroneous measurement.