A thin-film transistor including a channel layer being formed of an oxide semiconductor transparent to visible light and having a refractive index of nx, a gate-insulating layer disposed on one face of the channel layer, and a transparent layer disposed on the other face of the channel layer and having a refractive index of nt, where there is a relationship of nx>nt. A thin-film transistor including a substrate having a refractive index of no, a transparent layer disposed on the substrate and having a refractive index of nt, and a channel layer disposed on the transparent layer and having a refractive index of nx, where there is a relationship of nx>nt>no.