A case study is made of the formation of a meso-β-scale convective system that constitutes a part of a cloud cluster. The convective system was formed near a relatively small island in the subtropical ocean. The evolution process and the fine structure of the convective system were investigated by using mainly Doppler radar data. The evolution process of the convective system was divided into two stages: stationary stage (stage I) and propagating stage (stage II). Convective-scale circulations predominated in stage I, while a mesoscale circulation, similar to that found in squall lines, predominated in stage II. A thick layer of rear inflow was one of the characteristic features in the mesoscale circulation, and the rear inflow contributed to the formation of a severe gust front that exceeded 25 m s−1. The total area of strong echoes remained nearly constant after the mesoscale circulation was established. It is suggested that the squall-line-type circulation is important to maintain this strong convective system. Depending on the differences of internal circulation between stage I and stage II, the features of cells that constituted the convective system changed to an organized multicellular type in stage II.