An engine lubrication system in which a transmission 13 having a main shaft 14 and a drive shaft 15 each having a plurality of change-speed gears mounted thereon is arranged within a separate-type crankcase which is separated into left and right case portions, characterized in that a connecting bolt 60 for connecting the left and right case portions 2a, 2b together is inserted to be disposed in the vicinity of the main shaft 14 or the drive shaft 15 and in parallel with the shafts, in that left and right boss portions 60c having bolt bores 60a into which the connecting bolt 60 for connecting the left and right case portions 2a, 2b together is inserted are extended in a tubular fashion so as to abut with each other face-to-face, in that a space between an inner circumferential surface of the bolt bore 60a in the left and right boss portions 60c and an outer circumferential surface of the connecting bolt 60 is made to constitute a lubricating oil passageway 15a, in that an end of the lubricating oil passageway 15a is connected to a lubricating oil supply source, and in that lubricating oil supply holes 60b directed to the change-speed gears of the transmission are formed in the boss portions 60c in such a manner as to branch off from the lubricating oil supply passageway 15a.