NOVELTY - The device (1) has the input and output rotors (5,6) that are provided on the drive and output shafts (41,35) of a power generator (4) and a speed increaser (3) to rotate together with the drive and output shafts respectively. A one-way clutch (7) connects both the rotors together when the rotation speed of the input rotor is higher. The clutch disconnects both the rotors from each other when the rotation speed of the input rotor is lower. A reverse rotation prevention device (20) transmits force to the input rotor when a main shaft (2) rotates in a forward direction.    USE - Power generation device such as wind turbine generator. Can also be used for generating power using tidal power, water power and fire power.    ADVANTAGE - Since the one-way clutch disconnects the input rotor and the output rotor from each other when rotation speed of the input rotor is lower than rotation speed of the output rotor, the transmission of inertial rotation of the rotor of the power generator to the output shaft can be prevented. Hence the reduction in radial load acting on the roller bearing is restrained. Thus the smearing is restrained from occurring in the roller bearing. Since the rotor of the power generator continues to rotate by inertia without sudden speed drop, the average rotation speed of the rotor is increased. Thus power generation efficiency of the power generator is improved. Since the gears of the planet gear mechanism, each of the shafts and roller bearings can be downsized, the weight of the speed increaser can be reduced. Hence the speed increaser can be manufactured at the low cost. Since the reverse rotation prevention device can prevent the rotation of the main shaft and blades when the main shaft tries to rotate in a reverse direction, the backlash of the input rotor and the output rotor in a radial direction is prevented during an operation of the wind turbine generator.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic side view of the wind turbine generator.    Power generation device (1)    Main shaft (2)    Speed increaser (3)    Power generator (4)    Input rotor (5)    Output rotor (6)    One-way clutch (7)    Reverse rotation prevention device (20)    Output shaft (35)    Drive shaft (41)