1. An ultrasonic sonar for detecting an obstacle comprising: an ultrasonic oscillator for transmitting an ultrasonic wave and receiving an ultrasonic wave reflected from the obstacle, the ultrasonic oscillator having a reverberation frequency; and processing means for causing the ultrasonic oscillator to transmit the ultrasonic wave at a transmission frequency different from the reverberation frequency, converting an analog signal received by the ultrasonic oscillator into a digital signal on detection of the obstacle, and extracting only a component of the transmission frequency from the converted signal by digital signal processing.