In a process of manufacturing elements of different structures and characteristics on the same substrate at the same time, the number of steps is increased and complicated. In view of this, the invention provides a semiconductor device and a manufacturing process thereof in which elements of different structures are formed on the same substrate while reducing the number of steps. According to the invention, in accordance with a memory transistor that requires the largest number of steps when being formed among elements that forms a semiconductor memory device, other high speed transistor and high voltage transistor are efficiently manufactured. Thus, the number of steps is suppressed and a low cost semiconductor memory device can be manufactured.