A bearing for a wind turbine generator, such as a main shaft bearing that supports a shaft transmitting a blade's rotational power, is acted on not only by a load component attributed to the weight of the blade and that of a rotor but also by a load component attributed to a wind load. That is, in addition to a radial load, an axial load also acts on the bearing. For this reason, it has been conventionally proposed to use a double row tapered roller bearing as a bearing for a wind power generator (see, for example, Japanese Patent National Publication No. 2008-546948).