A semiconductor device includes a thin film transistor, wherein: a semiconductor layer of the thin film transistor has a layered structure including a lower oxide semiconductor layer including In, Ga, Zn and Sn and an upper oxide semiconductor layer arranged on the lower oxide semiconductor layer and including In, Ga and Zn; a thickness of the lower oxide semiconductor layer is 20 nm or less; an atomic ratio of Sn with respect to all metal elements of the lower oxide semiconductor layer is 5% or more; the upper oxide semiconductor layer includes no Sn, or an atomic ratio of Sn with respect to all metal elements of the upper oxide semiconductor layer is smaller than an atomic ratio of Sn with respect to all metal elements of the lower oxide semiconductor layer; and a first angle θ1 between a side surface and a lower surface of the lower oxide semiconductor layer is smaller than a second angle θ2 between a side surface and a lower surface of the upper oxide semiconductor layer.