A field effect transistor which includes, on a substrate, at least a semiconductor layer, a passivation layer for the semiconductor layer, a source electrode, a drain electrode, a gate insulating film and a gate electrode, the source electrode and the drain electrode being connected through the semiconductor layer, the gate insulating film being present between the gate electrode and the semiconductor layer, the passivation layer being at least on one surface side of the semiconductor layer, and the semiconductor layer including a composite oxide which comprises In (indium), Zn (zinc) and Ga (gallium) in the following atomic ratios (1) to (3): In/(In+Zn)=0.2 to 0.8  (1) In/(In+Ga)=0.59 to 0.99  (2) Zn/(Ga+Zn)=0.29 to 0.99  (3).