The purpose of this article is to ajnalyse patterns of Shipbuilding capital's accumulation during the First World War and to inquire into the turning point of monopoly system of shipbuilding industry in Japan. To accomplish this purpose, we must adapt the method of analysis to be able to analyse the movement of capitals. From this point of view, the conditions of capital accumulation must be exactly given to the rightful positions on the accumulation process of shipbuilding capitals. The conditions of capital accumulation of shipbuilding industry during this period is divided into two factors, one is what conditioned the development of shipbuilding industry and the other what restricted it. The former was the expansion of the market caused by the war. The development of trampers was its main factor. It was led to by the unbalance of demand and supply in the shipping market. As it continued during this period and European countries had no reserve power of export, trampers intented to raise vessels in the domestic market. As this result the market of shipbuilding industry expanded rapidly and the prices of ships raised very high level. These circumstances not only guaranteed the development of the established shipbuilding capitals but also stimulated the participation of the rising capitals. The development of rising capitals and established capitals exclusive of Mitsubishi, Kawasaki Zosen demolished in some degree the precocious monopoly system which had been established before the First World War. The latter was the lack of steel and skilled labourers. These conditions were curbed from the Japanese economic backwardness. To put it in the concrete, its economic backwardness means underdevelopment of iron and steel industry and the thin stratum of skilled labourers in the heavy industry in this article. The lack of steel and skilled labourers might restrict the accumulation of established capitals, but these capitals formed huge profits. They, however, acted as a barrier of participation for rising capitals. For that reason, monopoly system was supported by these conditions. In addition to it, Mitsubishi, Kawasaki Zosen dominated the market of warships as before. Therefore, on the one hand the patterns of shipbuilding capitals' accumulation during the First World War demolished the precocious monopoly system, on the other hand main tained it. In conclusion, it conditioned the formation of real monopoly system of shipbuilding industry during the nineteen-twenties.