The output of a photovoltaic power generator is estimated by estimating a solar radiation amount at a point different from solar radiation measurement points and determining an estimation error amount from solar radiation values measured at limited points. A photovoltaic power generator output estimation method estimates an output of a photovoltaic power generator based on a measured solar radiation value. The method includes a first estimation method of estimating a solar radiation amount at a photovoltaic power generator installation point from solar radiation meter installation point information, a value measured by a solar radiation meter, and photovoltaic power generator installation point information; a second estimation method of estimating the solar radiation amount at the photovoltaic power generator installation point in a manner different from that of the first estimation method; an output estimation method of estimating the output of the photovoltaic power generator from an estimated solar radiation amount and a rated capacity of the photovoltaic power generator; and an error estimation method of determining an estimation error of a photovoltaic power generator output from a difference between the first and second estimation methods.