A distributed-generator power-generation-amount estimation apparatus and method capable of more accurately and easily estimating output by reflecting a power factor are disclosed. The apparatus estimates a power generation amount of a distributed generator interconnected to an electric power system from active power and reactive power measured at regular intervals by measurement means, the apparatus including power-factor estimation means for estimating, as a ratio of a time change amount of the active power and the reactive power, a power factor of the distributed generator interconnected to a demand side of the electric power system, distributed-generator power-generation-amount estimation means for having, from active power and reactive power measured at a time of no power when the distributed generator interconnected to the electric power system is not generating power, a first characteristic line representing a load characteristic on a coordinate plane based on active power and reactive power, a second characteristic line representing a distributed-generator characteristic on the coordinate plane, and estimating the difference between the active power and the reactive power at an intersection point of the first characteristic line and the second characteristic line, and the measured active power and the measured reactive power as the power generation amount of the distributed generator, and output means for outputting power-generation-amount estimation information on the distributed generator estimated by the distributed-generator power-generation-amount estimation means.