PURPOSE: To enable the detection of an obstacle good in response and accuracy in a short distance region, by shortening a transmission cycle when an approach obstacle is detected.

CONSTITUTION: When a detection mode is changed over to a short distance obstacle detection mode 1, a control circuit 7 generates only a timing signal E at a cycle T1 and no timing signal F is generated. A generator circuit 5 generates a signal with a pulse amplitude t1 in synchronous relation to the rising of the signal E. With this signal generation, a transmission circuit 3 and a transmitter 1 are operated corresponding to the output G of an OR gate 4 and an ultrasonic wave is emitted at a cycle T1 in transmission amplitude t1. In addition, a signal I is generated by a receiver 2, an amplifier 8 and a shaping circuit 9 while a discriminator circuit 10 detects only the signal from a short distance obstacle and detects a time τ1 from the rising of the signal E to the rising of the signal I to discriminate the distance to the obstacle and the position of the obstacle is displaced by a display device 11 and an alarm 12. On the other hand, the detection of a long distance obstacle is performed by sending out ultrasonic waves each long in a transmission time at long transmission intervals.