A field effect transistor including: a substrate, and at least gate electrode, a gate insulating film, a semiconductor layer, a protective layer for the semiconductor layer, a source electrode and a drain electrode provided on the substrate, wherein the source electrode and the drain electrode are connected with the semiconductor layer therebetween, the gate insulating film is between the gate electrode and the semiconductor layer, the protective layer is on at least one surface of the semiconductor layer, the semiconductor layer includes an oxide containing In atoms, Sn atoms and Zn atoms, the atomic composition ratio of Zn/(In+Sn+Zn) is 25 atom % or more and 75 atom % or less, and the atomic composition ratio of Sn/(In+Sn+Zn) is less than 50 atom %.