To provide a crank shaft capable of preventing a fatigue failure to be generated in a connecting end and seizure of a crank pin and a journal part.

A curvature radius of a fillet part 7 formed in a connecting end of a crank pin 2 and an arm part 3 is formed larger at the axis center side of a journal part 4, and a curvature radius of a fillet part 7 formed in a connecting end of the journal part 4 and the arm part 3 is formed larger at a side close to the crank pin 2. Strong tensile force to be generated at a part B and a part C is relaxed by the fillet part 7 having a relatively large curvature radius, and the generation of fatigue failure is prevented. A part of the crank pin 2 near a surface A is strongly pushed by a connecting rod 5, but a contact area of the crank pin 2 with a bearing 6 is large at this pushed part, the generation of seizure is prevented.