In view of the problem that an organic semiconductor layer of an organic TFT is likely to deteriorate due to water, light, oxygen, or the like, it is an object of the present invention to simplify a manufacturing step and to provide a method for manufacturing a semiconductor device having an organic TFT with high reliability. According to the invention, a semiconductor layer containing an organic material is formed by patterning using a mask, and thus an organic TFT is completed in the state where the mask is not removed but to remain over the semiconductor layer. In addition, a semiconductor layer can be protected from deterioration due to water, light, oxygen, or the like by using the remaining mask.