A speed reducer used for a wind power generation facility includes a two-stage reduction mechanism of at least a front stage reduction mechanism and a rear stage reduction mechanism; a coupling that is arranged between the front stage reduction mechanism and the rear stage reduction mechanism and has a torque limiter mechanism which does not transfer a torque exceeding a predetermined value; and an oil seal that partitions and seals a space where the front stage reduction mechanism is accommodated and a space where the coupling is accommodated. The front stage reduction mechanism is separable from the coupling in a sealed state by the oil seal.