Initial Nd and Sr isotopic ratios have been measured for Cretaceous acidic and related intermediate rocks (24 volcanic and two plutonic rocks) from the Inner Zone of Southwest Japan (IZSWJ) to investigate the genesis of acidic magmas. The initial Nd and Sr isotopic ratios for these rocks show three interesting features: (1) eNd values for acidic rocks (+2 to −9) are negatively correlated with eSr values (+10 to +90) together with those for intermediate rocks (eNd=+3 to -8; eSr=0 to +65). (2) The eNd values for silica rich rocks (>60% SiO2) correlate with the longitude of the sample locality, decreasing from west to east in a stepwise fashion: Four areas characterized by uniform eNd values are discriminated. (3) Low silica rocks (<60% SiO2) in a certain area have distinctly different eNd values from those of the high silica rocks in the same area.