To provide a semiconductor device of high reliability by arranging TFTs that have appropriate structures in accordance with circuit functions. In a semiconductor device having a driver circuit portion and a pixel portion on the same insulator, gate insulating films of a driver TFT are designed to be thinner than a gate insulating film of a pixel TFT. In the pixel TFT, channel forming regions are formed under a gate electrode, and a separation region is formed between the channel forming regions. At this point, LDD regions have a region that overlaps with the gate electrode and a region that does not.