Abstract: This invention relates to an improved vertical swinging-blade water wheel for converting a kinetic energy of a substantially horizontal stream into mechanical energy in which a pair or pairs of blade wheels are disposed symmetrically with respect to an output shaft, thereby forming a composite water wheel and the water wheel itself as a whole is prevented from rotating. Further, a plurality of blade wheels vertically arranged with the phases of the blades deviated through a predetermined angle may prevent interference of streams between the blades and equalize the fluctuations of the driving torques, thereby improving the efficiency and securing a relatively large power output by using a small-sized blade wheel with a small number of blades. Since the water wheel itself will not rotate as a whole, it may be used as a power source for light buoy generators floating in the water and underwater pumps.