An anodic oxide film of InP, which had an interface state density of ≲ 1011 cm−2 eV−1 near midgap and worked well as the gate insulator for InP MOSFETs, was obtained by optimising its preparation conditions. The excellence of the anodic oxide as a gate insulator was confirmed by a high electron effective mobility (1500 cm2/Vs) in the accumulation-mode InP MOSFETs.