A hydroelectric power generation system comprising: a fluid machine arranged in a channel through which a fluid flows; a generator driven by the fluid machine; a controller (20, 30) configured to control at least one of electric power generated or electric power to be generated by the generator and to supply electric power generated by the generator to an electric power system; an electric power information acquisition unit configured to acquire power supply-and-demand information including electric power acceptable to the electric power system or information correlated with the electric power; and a fluid information acquisition unit (17, 18) configured to acquire fluid information including information correlated with a physical quantity of the fluid flowing out of the channel, wherein the controller (20, 30) is configured to control at least one of the physical quantity of the fluid flowing out of the channel, or the electric power generated or the electric power to be generated by the generator by using the fluid information so that the physical quantity is maintained at a desired value, while controlling electric power to be supplied to the electric power system to the electric power acceptable to the electric power system or less, by using the power supply-and-demand information, characterized in that the controller (20, 30) estimates a flow rate and an effective head in the fluid machine based on a detectable characteristic that is related to the generator and correlated with the flow rate and the effective head in the fluid machine, and estimates the total flow rate based on a flow resistance characteristic line representing a relation between the effective head and the total flow rate in the channel, the flow rate which has been estimated, and the effective head which has been estimated. a fluid machine arranged in a channel through which a fluid flows; a generator driven by the fluid machine; a controller (20, 30) configured to control at least one of electric power generated or electric power to be generated by the generator and to supply electric power generated by the generator to an electric power system; an electric power information acquisition unit configured to acquire power supply-and-demand information including electric power acceptable to the electric power system or information correlated with the electric power; and a fluid information acquisition unit (17, 18) configured to acquire fluid information including information correlated with a physical quantity of the fluid flowing out of the channel, wherein the controller (20, 30) is configured to control at least one of the physical quantity of the fluid flowing out of the channel, or the electric power generated or the electric power to be generated by the generator by using the fluid information so that the physical quantity is maintained at a desired value, while controlling electric power to be supplied to the electric power system to the electric power acceptable to the electric power system or less, by using the power supply-and-demand information, characterized in that the controller (20, 30) estimates a flow rate and an effective head in the fluid machine based on a detectable characteristic that is related to the generator and correlated with the flow rate and the effective head in the fluid machine, and estimates the total flow rate based on a flow resistance characteristic line representing a relation between the effective head and the total flow rate in the channel, the flow rate which has been estimated, and the effective head which has been estimated.