A water wheel impeller blade type electric power generating apparatus including a floating body, which buoys an attached electric power generating apparatus body while floating under water and allowing inflow and outflow of a water stream, an anchorage unit anchoring the electric power generating apparatus body at a fixed position in the water stream, a rotor pivotally supported within the electric power generating apparatus body enabling rotation, the rotor orthogonally disposed to the water stream, an impeller blade group including a plurality of impeller blades attached on an outer periphery of the rotor for automatically opening at a lower side of the rotor and closing at an upper side of the rotor, the impeller blade group receiving force by the water stream flowing in the electric power generating apparatus body with open impeller blades to rotate the rotor, an inclined plate for guiding the water stream, having one end attached at an upper end of the electric power generating apparatus body at a water inflow side with a projecting tip approaching a vicinity of an impeller blade attached to the rotor, and a power generator generating electric power by transmission of rotating force generated by rotation of the rotor, and an inclined plate set between 20° and 45° having a projecting tip set within a range of ±30% of a diameter of the rotor above or below a position corresponding to a shaft center of the rotor.