History, demonstrating convincingly that pirates have arisen continuously for a lengthy period of time throughout the world, is able to become a cooperative study of international law in terms of pirates matters ; Viking pirates. There are beneficial topics for the study of true nature of viking activities and the settlement of present pirates matters ; How were the pirates activities of Vikings, What sort of relations do they have between Vikings and other pirates which have arisen in world history, What are the differences compared to present concept of pirates. There were active pirates activities in the coast and waters of Scandinavia even before the period of the Migration Age because of geographical condition. With those experiences, Vikings began to ambush Britain Islands sailing across the North Sea since the late 8C, ages of migration in earnest. They ambushed all coasts of the European Continent expending boundary until the late of 11C. Pirate activities in a sort of guerrilla operations were operated when they encountered Islams in the Iberian Peninsula and the coast of North Africa. They showed twofold attitudes ; if the defence of the region and sea was weak, they plundered, or if strong, traded. In plundered europeans' position, Vikings were pirates with cruelty and barbarians. In vikings position, they were normal human beings who did a pirate activity to lead a better life. Viking pirates showed different characteristics in terms of three aspects ; area and aspect of action, activity after piracy. Meanwhile, Viking pirates showed several differences with pirates defined in terms of modern international law. Among the satisfying conditions of pirates, required by the international law of the sea, Vikings fulfilled animus furandi, desire for gain, activities for hatred and revenge, and private ends. Other conditions including attacking authority of the vessels, activities toward private ships, activities in the coast and the land, and illegal terroristic activities toward ships are found in viking pirates. However, Viking pirates do not show the activities in high seas and in the outside of a State's jurisdiction. In addition, it cannot be excluded that they pirated with vessels of regional leaders and the Sovereign, not private ships. Contrary to the definition of concept in terms of modern international law toward pirates, Viking invaded foreign waters, came on shore to foreign land and island, went up-stream the rivers to the back of interior, and attacked churches and abbeys. Strangely, they sometimes settled down in the places where they had pirated. Today, pirates appearing in history and defined in international law exist simultaneously and separately. It means, the historical nature and the nature under the international law are turning up differently. Historical cases of pirates should be reflected to modern international law. If so, it seems that the clue to solve pirate problems can be arranged. History is the immortal living thing, which not just existed as a past but reflects present.