A method for manufacturing a thin-film transistor includes: preparing a substrate; forming a gate electrode above the substrate; forming a gate insulating layer above the gate electrode; forming a semiconductor film above the gate insulating layer; forming, above the semiconductor film, a protective layer comprising an organic material; forming a source electrode and a drain electrode which are opposed to each other and each of which has at least a portion located above the protective layer; forming a semiconductor layer patterned, by performing dry etching on the semiconductor film; and performing, in a hydrogen atmosphere, plasma treatment on an altered layer which (i) is a surface layer of the protective layer exposed from the source electrode and the drain electrode and altered by the dry etching, and (ii) has at least a portion contacting a surface of the semiconductor layer.