The present invention provides a method of fabricating a TFT structure by two masking processes. More specifically, a light shielding layer and an interlayer insulating layer are sequentially formed on a substrate, and then source/drain electrodes are formed on the interlayer insulating layer (a first masking step). A semiconductor layer, a gate insulating layer and a gate metal layer are sequentially formed so as to cover the source/drain electrodes, and a gate electrode is formed in a second masking step. Subsequently, the gate insulating layer and the semiconductor layer are etched, and the interlayer insulating layer and the light shielding layer, which are disposed under the source/drain electrodes, are etched using the source/drain electrodes as a mask, thus obtaining a top gate TFT structure. When the interlayer insulating layer and the gate insulating layer are made of an insulating material containing SiOX and SiNX as a main component, the gate insulating layer and the semiconductor layer are naturally over-etched more than the interlayer insulating layer and the light shielding layer by plasma-etching with mixed gas of CF4 and hydrogen, thus obtaining a TFT structure with a high reliability, which is free from a problem of occurrence of photo-induced leak current.