A thin-film semiconductor device includes a gate electrode formed above a substrate; a gate insulating film formed to cover the gate electrode; a semiconductor layer formed above the gate insulating film and having a channel region; a channel protective layer formed above the semiconductor layer and containing an organic material which includes silicon, oxygen, and carbon; an interfacial layer which is formed in contact with the channel protective layer between the semiconductor layer and the channel protective layer, and which includes carbon as a major component, the carbon originating from the organic material; and a source electrode and a drain electrode which are electrically connected to the semiconductor layer.