A sequenctial-type synthetic aperture ultrasonic flaw dection technique has been investigated, which shows high resolving power in the flaw detection in steel. The computer simulation for image reconstruction by this technique showed that the lateral resolution of one wavelength can be attained when the aperture angle is about 90 degree and one-cycle burst signal is used. Based on the simulation results an experimental system for applying this technique has been developed. In this system the diverging-beam transducer connected to an ultrasonic flaw detector is moved along the object surface, and the received signals can be formed into a sectional image within one minute by using a newly developed echo location component. As predicted in the simulation, the experiments on the steel block specimens verified the capability of the system to reconstruct the sectional images of flaws with the lateral resolution of about 1mm (at 5MHz) regardless of the distance to the tested region. By using this technique the accuracy of flaw evaluation can be improved in ultrasonic nondestructive testing