To improve durability to fatigue breakage of a crank shaft in a cross head-type large multi-cylinder two-stroke diesel engine.

A crank shaft for a reciprocating piston engine includes a plurality of throws 2 interconnected by main journals 5. The main journals 5 have a main journal bearing surface 40 with a width W between two axial ends 42, 43 of the main journal bearing surface 40. At least one receding area 45 is provided in the main journal bearing surface 40 of at least one of the main journals 5. At least one receding area 45 adjoins one of the two axial ends 42, 43, and has a depth y that gradually decreases in the axial direction away from the one of the two axial ends 42, 43.