Accurately predicting a power generation of a natural energy power generation device that is affected by a peak cut loss. A processor of a computer is caused to execute: an insolation analysis step and a power generation prediction step. The power generation prediction step determines a first insolation that is the insolation required to obtain a first power generation that is zero and the minimum output power, and a second power generation that is the capacity. The power generation prediction step also determines a second insolation that is the insolation in a case where the capacity is satisfied. In a case where the insolation is lower than the first insolation, the power generation prediction step determines the first power generation as the power generation, and in a case where the insolation exceeds the second insolation, the power generation prediction step determines the second power generation as the power generation.