A manufacturing process OF a thin film transistor is provided, in which occurrence of a dry spot and occurrence of an etch residue of an ohmic contact layer (nnulla-Si:H film) due to the dry spot are prevented in photoengraving process for patterning a semiconductor layer and the ohmic contact layer into an island, without any further treatment by any other apparatus. After forming the a-Si:H film 4a which forms the semiconductor layer of the TFT and the nnulla-Si:H film 5a which forms the ohmic contact layer, a N2 gas plasma discharge is continuously performed using the same plasma CVD apparatus, thereby forming a very thin silicon nitride film 6 having a hydrophilic property on a surface layer of the nnulla-Si:H film 5a.