To understand the mode of differentiation of the granitic magmas of Southwest Japan, neutron activation analyses have been made to measure the REE abundance of the granitic rocks. We employed the Rayleigh fractionation model to simulate the crystallization of the granitic melts. Since the model calculations fit the observed REE data for the granitic rocks well, it is suggested that the granitic rocks of Southwest Japan were mainly differentiated by fractional crystallization. On the basis of the model calculations, it is also suggested that the granitic rocks with a large negative Eu anomaly found near some tungsten deposits are residues of an extremely differentiated magma.