To ensure lubricating performance of sliding surfaces where a crank pin and connecting rods contact with each other without reducing stiffness of a crank shaft when two connecting rods are supported by a common crank pin.

First branching oil passages 32L, 32R, expanding in a V-like manner in the direction away from an axial line Lj of a crank journal 22, are made to branch off from a slant oil passage 26 stretching slantwise from the crank journal 22 to an interior portion of the crank pin 21, lubricating the sliding surface contacting with the first connecting rod 19 using oil issued from two oil outlets 32a of the above two oil passages. Further, second branching oil passages 37L, 37R, expanding in a V-like manner in the direction nearer to the axial line Lj of the crank journal 22, are made to branch off from the same as described above, lubricating the sliding surface contacting with the second connecting rod 20 using oil issued from two oil outlets 37a of the above two oil passages.