A method of manufacturing a thin-film transistor according to an embodiment of the present invention includes the step of forming a gate insulator on a gate electrode. The gate insulator includes at least a first region being in contact with a hydrogenated amorphous silicon film, and a second region positioned below the first region. The first and second regions are deposited using a source gas including NH3, N2, and SiH4, and H2 gas or a mixture of H2 and He. The first region is deposited by setting the flow-rate ratio NH3/SiH4 in a range from 11 to 14 and the second region is deposited by setting the flow-rate ratio NH3/SiH4 to be equal to or less than 4. It is thus possible to provide a thin-film transistor having excellent characteristics and high reliability, a method of manufacturing the same, and a display device including the thin-film transistor mounted thereon.