from the oil supply path 50 both the oil supply paths and the piston jet branched oil passages 54 are cooled by outside air a surface area of the oil passages is increased by forming the plurality of piston jet branched oil passages 54 thus oil is brought into contact with and is cooled by inner surfaces of the oil passages and therefore the elevation of the oil temperature in the oil supply path 50 and the piston jet branched oil passages 54 can be suppressed journal lubrication oil passages 56 which extend toward the journals 55 formed in the crankshaft 10 of the internal combustion engine 1 are formed such that the journal lubrication oil passages 56 are branched from three portions of the oil supply path 50 and traverse the inside of the breather device 30 downward respectively upper half portions of the journal lubrication oil passages 56 are formed in the left and right side walls 34 of the breather device 30 and in the labyrinth walls 40 at the center portion of the breather device 30 in addition lower half portions of the journal lubrication oil passages 56 are formed on an outer wall body 60 of the crankcase 2 and a partition wall 61 formed between.