To provide a semiconductor device with a TFT, capable of reducing the electric resistance of a power supply wiring without increasing the off-current. The semiconductor device includes an insulating film with a surface; a semiconductor layer which is formed over the surface of the insulating film and which includes a channel region and a pair of source/drain regions and sandwiching the channel region; and a power supply wiring for supplying power to the source region. A concave portion is formed in the surface of the insulating film. The power supply wiring includes a layer formed from the same layer as the semiconductor layer, and has a first portion formed over the surface of the insulating film and a second portion formed in the concave portion. The bottom of the second portion is covered with an insulator.