A field effect transistor (FET) with high withstand voltage and high performance is realized by designing a buffer layer structure appropriately to reduce a leakage current to 1×10−9 A or less when a low voltage is applied. An epitaxial wafer for a field effect transistor comprising a buffer layer 2, an active layer, and a contact layer on a semi-insulating substrate 1 from the bottom, and the buffer layer 2 includes a plurality of layers, and a p-type buffer layer composed of p-type AlxGa1−xAs (0.3≦x ≦1) is provided as a bottom layer (undermost layer) 2a. A Nd product of a film thickness of the p-type buffer layer and a p-type carrier concentration of the p-type buffer layer is within a range from 1×1010 to 1×1012/cm2.