A shaft coupling device integrally rotatably connects an output shaft of a speed-up gear and an input shaft of a power generator to each other in a wind power generation device. The shaft coupling device includes a one-way clutch which is disposed between an inside rotating body and an outside rotating body along a radial direction, which integrally rotatably connects the inside rotating body and the outside rotating body to each other in a state in which a rotational speed of the output shaft exceeds a rotational speed of the input shaft, and breaks connection between the inside rotating body and the outside rotating body in a state in which the rotational speed of the output shaft is lower than the rotational speed of the input shaft.