A field-effect transistor which comprises a buffer layer and a barrier layer each of which is made of a Group III nitride compound semiconductor and has a channel at the interface inside of the buffer layer to the barrier layer, wherein the barrier layer has multiple-layer structure comprising an abruct interface providing layer which composes the lowest semiconductor layer in said barrier layer and whose composition varies rapidly at the interface of said buffer layer, and an electrode connection plane providing layer which constructs the uppermost semiconductor layer and whose upper surface is formed flat.