In the Japanese Late Paleozoic group basaltic volcanics are commonly found whose chemical natures are very similar to the abyssal basalt of the present mid-oceanic ridge, characterized, above all, by low K 2O, low Fe 3+/Fe 2+ and high K/Rb. Two types of the basaltic rocks are distinguished, especially in their distribution pattern of rare-earth elements, the one displaying the logarithmically rectilinear relationship of chondrite-normalized pattern and the other with the inflection point. Recently, Precambrian metamorphic cobbles dated at 1,700-1,500 m.y. have been found in Permian conglomerates of central Japan. Paleocurrent data indicate lateral filling of clastic materials derived from the north, where a vast expanse of Precambrian complex is supposed to have occurred. Stratigraphy and structure of the Paleozoic terrain of central Japan suggest that the Japanese Paleozoic geosyncline was grown on the Precambrian continental crust. From geochemical properties and the distribution of the geosynclinal basalt in the Japanese Islands, it is concluded that the basaltic rocks were derived from the upper mantle, the original magma having been created by partial melting of the mantle and moved upward through a rifting zone in the Precambrian crust. The early stage of tectonism of the geosynclinal belt was thought to be under a tensional condition rather than a compressional one.