Disclosed is a thin film transistor that is provided with a gate insulating film that is inexpensive, and that is less likely to have a low-density microcrystalline silicon layer formed thereon due to plasma induced damage, while suppressing fluctuation of a threshold voltage. In a TFT (100) having the bottom gate structure, since a silicon nitride film (31) having a natural oxide film (32) formed on the surface thereof is used as the gate insulating film (30), the gate insulating film (30) is not only capable of preventing the alkali metal ions contained in a glass substrate (10) from entering the gate insulating film (30), but also capable of suppressing a formation of the low-density microcrystalline silicon layer on the surface of a microcrystalline silicon film (41) on the side in contact with the gate insulating film (30). Since the mobility of the microcrystalline silicon film (41) is increased, the operation speed of the TFT (100) can be improved. Thus, with the simpler configuration, the TFT (100) having the same electrical characteristics as those of the conventional TFTs can be provided.