PURPOSE: To improve the discriminating effect of noise and raise a response speed by providing a counter to generate three kinds of pulse interval and making the first and the last pulse intervals of the transmission code equal to each other generating optional one or two pulse intervals as the other pulse intervals.

CONSTITUTION: An ultrasonic transmitter is provided with a pulse generating circuit 1a having a counter and a control circuit 1b, and an ultrasonic receiver provides a counter 8 which has a pulse interval observing counter and counters for pulse intervals B, C, and D. The first pulse interval B of transmission and the last pulse interval of transmission are made equal to each other, and optional one or two pulse intervals are used as the other pulse intervals. It is decided whether a pulse interval is within a range of the first or the last pulse interval or within a range of said optional one or two pulse intervals. If the puse interval is within the former or the latter, the counted value is stored in the counter for B in case of the first pulse interval. When the pulse interval is decided to be the last pulse interval, the counted value and contents of the counter for B are compared with each other to decide whether their difference is ±3msec or not.