A field-effect transistor includes a substrate; a source electrode, a drain electrode, and a gate electrode that are formed on the substrate; a semiconductor layer by which a channel is formed between the source electrode and the drain electrode when a predetermined voltage is applied to the gate electrode; and a gate insulating layer provided between the gate electrode and the semiconductor layer. The gate insulating layer is formed of an amorphous composite metal oxide insulating film including one or two or more alkaline-earth metal elements and one or two or more elements selected from a group consisting of Ga, Sc, Y, and lanthanoid except Ce.