The present invention relates to a thin film device. There is such an issue with a TFT using an oxide semiconductor film that oxygen deficit is generated in a surface region of the oxide semiconductor film after performing plasma etching of a source-drain electrode, and the off-current becomes increased. Disclosed is the TFT which includes: a gate electrode on an insulating substrate as a substrate; a gate insulating film on the gate electrode; an oxide semiconductor film on the gate insulating film; and a source/drain electrode on the oxide semiconductor film. It is the characteristic of the TFT that a surface layer containing at least either fluorine or chlorine exists in a part of the oxide semiconductor film where the source/drain electrode is not superimposed.