The squall line on 4 April 1959 at the Korea Straits is analyzed with the use of the surface observations from regular synoptic stations, upper air observations and PPI radar pictures. The analysis revealed that the mesohighs lay beneath the linear-echoes behind the squall line and they extended to the cold front, so that the original air that occupied the warm sector did not intervene into the space between the squall line or pressure surge line, and the cold front. Mesoanalysis of the upper wind, using a technique based on the concept that the time-section may be transformed into the space-section, proved that the upper air converged on such a lower level as 1000mb to 850mb level, and diverged on such a upper level as 500mb to 300mb level. It appears therefore that, this coupled with the thermal advection in the upper layer, and excited the production of thunderstorms in front of the cold front. Thermal and moisture advections from the sea surface and by the lower southerly current, however, were not so strong.