Thin-film semiconductor device and method of manufacturing the same

A thin-film semiconductor device manufacturing method according to the present disclosure includes: preparing a substrate; forming a gate electrode above the substrate; forming a gate insulating film above the substrate; forming an amorphous film (amorphous silicon film) above the substrate; forming a crystalline film (crystalline silicon film) including a first crystal and a second crystal, by crystallizing the amorphous film, the first crystal (i) containing subgrains formed with different crystal orientations in a single crystal and (ii) including a subgrain boundary formed by plural crystal planes between the subgrains, the second crystal having an average crystal grain size smaller than an average crystal grain size of the first crystal; thinning the crystalline film; and forming a source electrode and a drain electrode above the substrate.