Thin film transistors for a display device each include a semiconductor layer made of polysilicon having a channel region, drain and source regions at both sides of the channel region and doped with impurity of high concentration, and an LDD region arranged either between the drain region and the channel region or between the source region and the channel region and doped with impurity of low concentration. An insulation film is formed over an upper surface of the semiconductor layer and has a film thickness which decreases in a step-like manner as it extends to the channel region, the LDD region, the drain and the source regions; and a gate electrode is formed over the channel region through the insulation film. Such a constitution can enhance the numerical aperture and can suppress the magnitude of stepped portions in a periphery of the thin film transistor.