An ultrasonic diagnostic apparatus includes an ultrasonic transducer housed in a catheter for sending an ultrasonic wave to a blood vessel and receiving a reflected signal from the blood vessel, an A/D converter for converting the received reflected signal into a digital signal, a data averaging processor for storing lines or frames of digital data represented by the digital signal and adding the stored lines or frames of the digital data to average the reflected signal, a demodulating/logarithmic converting processor for demodulating the averaged reflected signal, an image constructor for constructing an image in a radial scan B mode from the demodulated signal, and a display unit for displaying the constructed image in the radial scan B mode. The ultrasonic diagnostic apparatus plots a cross-sectional vascular image with a reduced ultrasonic echo reflected from blood cells, the image clearly displaying the boundary between the blood vessel wall and the blood vessel lumen.