A thin-film semiconductor device including a transparent insulating substrate, an island semiconductor layer formed on the transparent insulating substrate and including a source region containing a first-conductivity-type impurity and a drain region containing a first-conductivity-type impurity and spaced apart from the source region, a gate insulating film and a gate electrode which are formed on a portion of the island semiconductor layer, which is located between the source region and the drain region, a sidewall spacer having a 3-ply structure including a first oxide film, a nitride film and a second oxide film, which are respectively formed on a sidewall of the gate electrode, and an interlayer insulating film covering the island semiconductor layer and the gate electrode.