An internal combustion engine, wherein an electric generator for generating electric energy upon rotation of a crankshaft supported in a crankcase of the internal combustion engine has a cup shaped outer rotor rotatable in unison with said crankshaft, said outer rotor being of a bottomed hollow cylindrical shape having a hollow cylindrical portion and a bottom wall, said bottom wall being fixed to an end of said crankshaft such that said hollow cylindrical portion has an opening directed outward along an axial direction of said crankshaft, and said electric generator has an inner stator having coils disposed in said outer rotor, wherein said coils are wound on a stator core and facing inner sides of magnets disposed on an inner circumferential surface of said hollow cylindrical portion of said outer rotor, comprising: a generator cover covering an axially outer side of said electric generator, said generator cover being of a modified cup shape having a peripheral wall and a side wall closing an opening of said peripheral wall, and said generator cover having a mating surface, to which said crankcase is mated, said mating surface including the end face of an edge surrounding said opening in said peripheral wall; 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 internal combustion engine toward said electric generator; an ejection port (Dj) defined in a downstream end of said first cooling oil passage and provided at a position for ejecting lubricating oil onto an outer side surface of said bottom wall of said cup shaped outer rotor and onto an outer circumferential surface of said hollow cylindrical portion thereof; wherein said second cooling oil passage includes a cooling oil passageway branched from a branch oil slot defined in said mating surface of said generator cover, wherein said cooling oil passageway extends from said branch oil slot obliquely in said peripheral wall and has an end crossing held in fluid communication with a lower end of a cooling oil passageway defined substantially vertically in said side wall of said generator cover; wherein said cooling oil passageway has an upper end crossing and is held in fluid communication with a rear end of a downstream-end cooling oil passageway defined obliquely forwardly and downwardly in said side wall, wherein said downstream-end cooling oil passageway overlaps an upper portion of said generator as viewed in side elevation; and wherein said downstream-end cooling oil passageway is positioned in facing relation to said inner stator and has an oil ejection port (Ej) to eject lubricant oil onto said inner stator. a generator cover covering an axially outer side of said electric generator, said generator cover being of a modified cup shape having a peripheral wall and a side wall closing an opening of said peripheral wall, and said generator cover having a mating surface, to which said crankcase is mated, said mating surface including the end face of an edge surrounding said opening in said peripheral wall; 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 internal combustion engine toward said electric generator; an ejection port (Dj) defined in a downstream end of said first cooling oil passage and provided at a position for ejecting lubricating oil onto an outer side surface of said bottom wall of said cup shaped outer rotor and onto an outer circumferential surface of said hollow cylindrical portion thereof; wherein said second cooling oil passage includes a cooling oil passageway branched from a branch oil slot defined in said mating surface of said generator cover, wherein said cooling oil passageway extends from said branch oil slot obliquely in said peripheral wall and has an end crossing held in fluid communication with a lower end of a cooling oil passageway defined substantially vertically in said side wall of said generator cover; wherein said cooling oil passageway has an upper end crossing and is held in fluid communication with a rear end of a downstream-end cooling oil passageway defined obliquely forwardly and downwardly in said side wall, wherein said downstream-end cooling oil passageway overlaps an upper portion of said generator as viewed in side elevation; and wherein said downstream-end cooling oil passageway is positioned in facing relation to said inner stator and has an oil ejection port (Ej) to eject lubricant oil onto said inner stator.