A method of manufacturing a semiconductor device is provided that can suppress impurity concentration reduction in a doped channel region arising from formation of a gate insulating film. With a silicon oxide film (20) and a silicon nitride film (21) being formed, p-type impurity ions (23.sub.1, 23.sub.2) are implanted in a Y direction from diagonally above. As for an implant angle .alpha. of the ion implantation, an implant angle is adopted that satisfies the relationship tan.sup.-1(W2/T)<.alpha..ltoreq.tan.sup.-1(W1/T), where W1 is an interval between a first portion (21.sub.1) and a fourth portion (21.sub.4) and an interval between a third portion (21.sub.3) and a sixth portion (21.sub.6); W2 is an interval between a second portion (21.sub.2) and a fifth portion (21.sub.5); T is a total film thickness of the silicon oxide film (20) and the silicon nitride film (21). When the implant angle .alpha. is controlled within that range, impurity ions (23.sub.1, 23.sub.2) are implanted into a second side surface (10A.sub.2) and a fifth side surface (10A.sub.5) through a silicon oxide film (13).