A thin-film transistor (1) of the present invention includes an insulating substrate (2), a gate electrode (3) which has a predetermined shape and is formed on the insulating substrate (2), a gate insulating film (4) formed on the gate electrode (3), and a semiconductor layer (5) which is polycrystalline ZnO and is formed on the gate insulating film (4). The semiconductor layer (5) is immersed in a solution in which impurities are dissolved so that the impurities are selectively added to a grain boundary part of the polycrystalline ZnO film. Subsequently, a source electrode (6) and a drain electrode (7) are formed so as to have a predetermined shape. Next, a protection layer (8) is formed on the source electrode (6) and the drain electrode (7). Thus, a thin-film transistor which has a good subthreshold characteristic and has a zinc oxide film as a base of an active layer can be realized.