A thin-film semiconductor device according to the present disclosure includes: a substrate; a gate electrode formed above the substrate; a gate insulating film formed on the gate electrode; a channel layer that is formed of a polycrystalline semiconductor layer on the gate insulating film; an amorphous semiconductor layer formed on the channel layer and having a projecting shape in a surface; and a source electrode and a drain electrode that are formed above the amorphous semiconductor layer, and a first portion included in the amorphous semiconductor layer and located closer to the channel layer has a resistivity lower than a resistivity of a second portion included in the amorphous semiconductor layer and located closer to the source and drain electrodes.