To attain superior workability while strength and durability are being maintained, eliminate the necessity of arranging a special rib and attain more lightening.

Lightening structure of a web 2 at a journal 4 of a crankshaft 1 is attained so that grooves 6 are provided which are continuously narrowed from an outer shell of the web toward a journal axis 0 with the journal axis 0 being held therebetween and their U-shaped bottoms being symmetrically arranged, wherein the depth (H) of each of the grooves 6 in respect to the web thickness W in a longitudinal direction of the crank shaft 1 is set to be 0.4 to 0.6 times.