A power generation device comprising: a main shaft rotated by external force; a speed increaser including a rotation transmission mechanism that increases speed of rotation of the main shaft and an output shaft that outputs rotation, speed of which has been increased by the rotation transmission mechanism a power generator including a drive shaft that rotates when receiving the rotation of the output shaft as an input, and a rotor that rotates together with the drive shaft, the power generator generating power with rotation of the rotor; an input rotor provided on the output shaft so as to be rotatable together with the output shaft; an output rotor that is provided on the drive shaft so as to be rotatable together with the drive shaft, and that is arranged radially inside or radially outside the input rotor concentrically with the input rotor; a clutch that is arranged between the input rotor and the output rotor, connects the input rotor with the output rotor so that the input rotor and the output rotor are rotatable together with each other when the main shaft rotates in a forward direction and rotation speed of the input rotor is higher than rotation speed of the output rotor, and disconnects the input rotor and the output rotor from each other when the rotation speed of the input rotor is lower than the rotation speed of the output rotor; characterized in that the speed increaser includes a roller bearing (38,39) that supports the output shaft so that the output shaft is rotatable and in that the power generation device comprises a reverse rotation prevention device that permits transmission of force, which is caused by the rotation of the main shaft, to the input rotor when the main shaft rotates in the forward direction, and restricts the rotation of the main shaft when the main shaft tries to rotate in a reverse direction; wherein the reverse rotation prevention device is a one-way clutch that restricts the rotation of the main shaft when the main shaft tries to rotate in the reverse direction. a main shaft rotated by external force; a speed increaser including a rotation transmission mechanism that increases speed of rotation of the main shaft and an output shaft that outputs rotation, speed of which has been increased by the rotation transmission mechanism a power generator including a drive shaft that rotates when receiving the rotation of the output shaft as an input, and a rotor that rotates together with the drive shaft, the power generator generating power with rotation of the rotor; an input rotor provided on the output shaft so as to be rotatable together with the output shaft; an output rotor that is provided on the drive shaft so as to be rotatable together with the drive shaft, and that is arranged radially inside or radially outside the input rotor concentrically with the input rotor; a clutch that is arranged between the input rotor and the output rotor, connects the input rotor with the output rotor so that the input rotor and the output rotor are rotatable together with each other when the main shaft rotates in a forward direction and rotation speed of the input rotor is higher than rotation speed of the output rotor, and disconnects the input rotor and the output rotor from each other when the rotation speed of the input rotor is lower than the rotation speed of the output rotor; characterized in that the speed increaser includes a roller bearing (38,39) that supports the output shaft so that the output shaft is rotatable and in that the power generation device comprises a reverse rotation prevention device that permits transmission of force, which is caused by the rotation of the main shaft, to the input rotor when the main shaft rotates in the forward direction, and restricts the rotation of the main shaft when the main shaft tries to rotate in a reverse direction; wherein the reverse rotation prevention device is a one-way clutch that restricts the rotation of the main shaft when the main shaft tries to rotate in the reverse direction.