A gas turbine washing-date determination apparatus of the present invention is equipped with a sum cost calculation means for calculating a sum of loss cost due to not washing a compressor from a compressor efficiency calculated, and determines a gas turbine washing-date with using the sum of the loss cost. Meanwhile, in the gas turbine washing-date determination apparatus the compressor efficiency is calculated from process data of a gas turbine plant, and a compressor washing-date is determined, based on the compressor efficiency.