Establishment processes of the mixed coniferous and deciduous broad-leaved forest containing such dominant conifers as Pinus koraiensis, Picea jezoensis and other Picea species of sect. Picea, which have occupied vast areas of Honshu in the Last Glacial age, was discussed focusing on the ecological role of Pinus koraiensis. The most prominent feature of the tree distribution in the Last Glacial age of Honshu was that Pinus koraiensis and Picea sect. Picea conifers occurred more abundantly than at present, and that the mixed Pinus koraiensis and deciduous broad-leaved forest prevailed mainly on the lowland of Honshu. To clarify the ecological role of Pinus koraiensis, the distribution and the species relationship were analyzed at the stands of the mixed Pinus koraiensis and deciduous broad-leaved forest along the river Anyui, northern Sikhote-Alin, Far East Russia. The existence of Pinus koraiensis prevented the deciduous broad-leaved species from their prevalence in the mixed forests, and promoted the abundant occurrence of Picea jezoensis within the potentially deciduous broad-leaved forest areas, resulting in the occurrence of the mixed Pinus koraiensis and deciduous broad-leaved forest. Furthermore, to understand the ecological characteristics of Picea sect. Picea trees, the tree form and the height growth were compared between Picea jezoensis (sect. Casicta) and Picea glehnii (sect. Picea) in the coniferous forests near the lake Akan, eastern Hokkaido. It was deduced from these comparison that on the mesic site Picea jezoensis dominated, while on the dry site Picea glehnii maintained its abundant occurrence. In the Last Glacial age Pinus koraiensis encouraged the prevalence both of Picea jezoensis and Picea sect. Picea trees, reducing the occurrence of deciduous broad-leaved trees. It was concluded that Pinus koraiensis played an important ecological role on the establishment of the mixed coniferous and deciduous broad-leaved forest.