In recent years, due to exhaustion of fossil fuels and environmental issues, wind power generation, which uses natural energy, has been receiving considerable attention.  Since wind power generation requires an increased power generation efficiency due to a low rotation speed of a rotor, a step-up gear is provided in a power generator.  A so-called gear oil is used to lubricate a gear mechanism used in the step-up gear, and is required to provide a considerably high lubricity. 
Typically, a lubricating oil whose base oil is PAO (polyalphaolefin) has been used as a step-up gear oil.  Since a wind power generator is frequently used on the ocean or under the natural environment, the step-up gear oil should be highly biodegradable.  The typical PAO lubricating oil, however, has little biodegradability, so that an alternative thereto has been sought for. 
As a lubricating oil intended to be used for a step-up gear in a wind power generator, a lubricating oil whose base oil is ester can be applicable because such a lubricating oil needs to be biodegradable (see, for instance, Patent Literatures 1 and 2).  Each of Patent Literatures 1 and 2 has suggested a biodegradable lubricating oil whose base oil is a complex ester obtained from a polyhydric alcohol and a polycarboxylic acid.