PURPOSE: To improve the effect of cooling a slide surface on the center side of a shaft in a crank pin section.

CONSTITUTION: In an internal combustion engine having a crankshaft 10 for converting the reciprocation of a piston to the rotation through a connecting rod, the crankshaft 10 is formed with an oil path 28 for lubricating oil which supplies the lubricating oil from the main bearing side of the shaft 10 to the slide surface of the connecting rod in crank pin sections 23-26. Separately from this lubricating oil path 28 is formed a cooling oil path 30 for releasing the lubricating oil from the main bearing side of the shaft 10 through a part near the slide surface of the connecting rod at the center side of the shaft in the crank pin sections 23-26 to the atmospheric pressure, and a lubricating oil controlling mechanism for adjusting an oil amount fed to the cooling oil path 30 is provided.