PURPOSE: To stabilize the detection and the distance measurement of a moving object and to obtain necessary resolving capacity and priciseness, by changing the repeating cycle of a transmission pulse by the value only corresponding to the number clock pulse at every transmission even in a constant measuring distance.

CONSTITUTION: In order to change the lower figure of a preset value in a transmission pulse generator 3 at every transmission at random, a ring counter 1' is provided and a clock pulse CL1 with a fast cycle not synchronous to the clock pulse from a generator 2 is imparted from a generator 2' and the lower figure of the preset value is changed at random prior to inputting the clock pulse from the generator 2 in response to measuring distance information by the output of a counter 1'. By this mechanism, the repeating cycle T of ultrasonic generation is changed in the cycle of the clock pulse corresponding to a distance at every transmission and further changed at random by the value only corresponding to the number clock pulse even in the constant distance thereof. The distance information of a counter 1 is displayed by a display device 12 and simultaneously sent to a pulse controller 1 to determine the next measuring distance range.