To provide a crank mechanism of a reciprocating engine capable of improving durability by preventing partial wear of a piston 31 and cylinder 33.

The crank mechanism of the reciprocating engine is comprised of arms 12 and 12 attached to main shaft 11 and 11, links 13 and 13 connecting the arms 12 and 12 with a connecting rod 32 of a piston 31, and a driving mechanism 20. The driving mechanism 20 rotatably drives the links 13 and 13 within a reverse direction around a connecting position with the arms 12 and 12 at double angular velocity against the arms 12 and 12. Upon maintaining the connecting rod 32 in a centerline C of the cylinder 33, the connecting rod 32 cannot be oscillated in proportion to reciprocating motion of the piston 31.