The echo signals from ultrasonic transducer elements are filtered by a plurality of prestage band pass filters having different central frequencies and frequency pass bands. The filtered echo signals from those band pass filters are composed by an adder into a composite echo signal. This composite echo signal from the adder is filtered by a variable band pass filter whose central frequency is shifted and whose frequency band is fixed, whereby the echo signal components passed through the prestage band pass filters are extracted sequentially. The echo signal components from the variable band pass filter are converted by a signal processing circuit into a tomographic signal, which is displayed as a tomographic image by a display.