An electric generator for generating electric energy upon rotation of a crankshaft supported in a crankcase of an internal combustion engine has an outer rotor and an inner rotor. An electric generator cooling structure includes a first cooling oil passage and a second cooling oil passage that are branched from a lubricating oil passage of a lubricating system of the engine toward the electric generator. A first ejection port is defined in a downstream end of the first cooling oil passage for ejecting lubricating oil to an outer side surface of the outer rotor, and a second ejection port is defined in a downstream end of the second cooling oil passage for ejecting lubricating oil to the inner stator disposed in the outer rotor. The electric generator cooling structure can thus efficiently cool the electric generator in its entirety.