It is the object of the invention to provide a method for fabricating a semiconductor device, such as a bipolar transistor, with improved characteristics when used in a semiconductor integrated circuit, without increasing the steps in fabricating process. In forming the graft base of the bipolar transistor, oxygen ions with higher energy than that of impurities are injected through the same mask. Thereafter, an insulating film is formed under the graft base region, by activating thermal treatment. Moreover, in a semiconductor integrated circuit of BiCMOS type, insulation films are formed under a source and a drain of a P-type transistor.