To use an almost all water flow of a natural river for power generation by utilizing the uneven river bed and allow the power generation even when there is a small amount of water in the river.

A natural river power generation system includes a water power generation device 10 which is installed on the river bed of a lower step portion of the natural river having steps in the river bed and across both banks, while packing both banks with concrete and setting its ceiling part flush with an upper step portion of the river. The water power generation device 10 includes: a waterfall 20 having a waterfall path 21 provided at the bottom center and a V-shaped waterfall slope 22 formed toward the waterfall path; at least one or a pair of water turbines 25 installed under the waterfall slope 22, receiving waterfall from the waterfall slope and converting the kinetic energy into rotating motion; a power generation chamber 30 continuously provided on the upstream side of the waterfall 20, in which a power generation device having transmission of the rotating motion of the water turbines 25 and generating electric power is installed; and a waterfall guide block wall 40 continuously provided on the downstream side of the waterfall 20 for guiding waterfall to the water turbines 25 and having a water discharge path 41 formed on the lower side in communication with the waterfall path 21.