The feralization of livestock is one of the themes of geography, but few studies have been done aside from those of T. L. McKnight. The author studied feral pigs, with particularr emphasis on distribution, time of the feralization, background of feralization, and attitudes toward feral pigs, through analysis of reference works in geography and other academic fields and investigation in Australia and the Ogasawara Islands in Japan. The author examined the distribution of feral pigs in comparison with that of Eurasian wild boars (from which pigs are domesticated). The distinction between feral pigs and Eurasian wild boars is not always clear, however. Domestic pigs roamed and had opportunities to become wild under traditional husbandry practices such as free ranging and mast feeding in Europe and Asia, and they are mostly regarded to have merged with Eurasian wild boars. Further, introduced Eurasian wild boars interbred with feral pigs in the United States, the Republic of South Africa, and parts of South America. Almost everywhere there is evidence of human modification of the indigenous fauna. Regarding the time of feralization, the author examined data from North and South America, Australia, and some islands where the time was comparatively clear. According to this analysis, pigs were introduced to some islands in the Pacific Ocean by Melanesians and Polynesians, and feralization occurred even though their distribution was limited. From the 15th century to the 19th century, explorers, colonists, sealers, and whalers introduced or released pigs on many oceanic islands and new continents. In this period, pig feralization occurred frequently, and the main feral pig's areas were established. Deliberate release by landholders and hunters continues in some areas even today. Extensive husbandry, deliberate release, and accidental escape are important factors in feralization. The spread of practices such as free ranging and mast, feeding contributed to pig feralization. Deliberate release by explorers, fishermen, landholders, and hunters is also a major factor. In these cases, we can point out the influence of humans who recognize the value of feral pigs as sources of food or game. Regarding attitudes toward feral pigs, the author examined data from Australia and the United States. In general, feral livestock including feral pigs were useful to pioneers and early settlers as supplementary animals. But as time passed, feral pigs came to be legally classified as noxious or verminous in Australia because of the damage they caused wheat farmers and raisers of sheep and cattle. In the United States, where such severe damage did not occur, feral pigs are legally classified as game. However, in both countries there are conflicts of interest among farmers, hunters, and landholders who collect hunting fees; all these groups have different attitudes toward feral pigs, which make it difficult to control feral pigs effectively. In the Ogasawara Islands, feral pigs were recorded as early as Commodore Perry's expedition in 1853. But it is difficult to collect clear evidence of these feral pigs today. In the period in which the islands under occupation by the U. S. Navy after World War II, pigs were brought from Tinian Island by the Navy, and a number of them were released. It is said that pigs were released for food supply and game on Ototo Island and Chichi Island, as a future source of food of fishermen on Muko Island and Nakodo Island. Their descendants are thought to have survived on Ototo Island.