A semiconductor device includes a substrate and a thin film transistor supported by the substrate. The thin film transistor includes a gate electrode, an oxide semiconductor layer, a gate insulating layer provided between the gate electrode and the oxide semiconductor layer, and source and drain electrodes electrically connected to the oxide semiconductor layer. The gate insulating layer includes a first portion which is covered with the oxide semiconductor layer and a second portion which is adjacent to the first portion and which is not covered with any of the oxide semiconductor layer, the source electrode and the drain electrode. The second portion is smaller in thickness than the first portion, and the difference in thickness between the second portion and the first portion is more than 0 nm and not more than 50 nm.