the fifth oil passage p5 therefore end openings of the two oil passages p5 and p6 form oil holes 13 and 14 which are oppositely located in the fifth balance weight b5 meanwhile former oil holes 11 12 15 and 16 are located such that the first oil hole 11 at the second balance weight b2 faces toward the second piston the second oil hole 12 faces toward the first piston and oil holes 15 and 16 at the eighth balance weight b8 face toward the fifth and sixth piston respectively further the newly formed third and fourth oil holes 13 and 14 are disposed toward the third and fourth piston respectively as described above the oil passages p1 p6 of the crankshaft 10 have six oil holes 11 16 formed as couples 11 and 12 13 and 14 and 15 and 16 at the second fifth and eighth balance weights b2 b5 and b8 respectively and each couple of oil holes at a balance weight are open to an opposite direction with respect to each other at the balance weight each oil hole 11 16 faces toward one of the six pistons not shown the detailed structure of an oil hole 11 of the balance weight b1 and b2 is illustrated in fig4 as an example the other oil holes 12 16 are similar in structure thereof to the illustrated one the first oil passage p1 is started from the first journal j1 and passes through the first weight balance weight b1 and the first crank pin c1 and terminates at the second weight balance b2 which has an oil hole 11 open at its right side an oil jet.