PURPOSE: To stably detect a body from a short distance to a long distance by decreasing the attenuation quantity of an attenuator provided behind a receiving amplifier circuit with the time from the transmission of an ultrasonic wave pulse.

CONSTITUTION: Right after the transmission of the ultrasonic wave pulse, namely, when a detection distance is short, the attenuation quantity of the attenuator 20 provided behind the receiving amplifier circuit 7 is made large in circuit constitution wherein the attenuation quantity of the attenuator 20 provided behind the receiving amplifier circuit 17 is variable. Then the attenuation quantity of the attenuator 20 is decreased stepwise or continuously to increase the detection rate of a long-distance body, thereby obtaining stable detection performance continuously from a short distance to a long distance.