forming a first semiconductor region of a first conductive type and a second semiconductor region of a second conductive type in a predetermined region of a semiconductor substrate of the first conductive type;

controlling threshold voltages corresponding to each transistor formed on the semiconductor substrate, the first semiconductor region and the second semiconductor region respectively by first to third ion implantation processes executed sequentially;

wherein the first ion implantation process is executed in a high-threshold low-voltage transistor forming region of the first semiconductor region after the forming the first semiconductor region; the second ion implantation process is executed in a high-threshold low-voltage transistor forming region of the second semiconductor region; and the third ion implantation is executed simultaneously in low-threshold low-voltage transistor forming regions of the semiconductor substrate and the second semiconductor region respectively.