To provide a stroke variable reciprocating device in which the compression ratio of piston reciprocating motion in reciprocating internal combustion engine can be continuously changed through a mechanism that is simple and small in increased dimensions and small in noises.

An eccentric cam member 5 is mounted on a crank pin 31 of a crank shaft 3. The eccentric cam member 5 includes an inner face 51 matching rotatably relative to the crank pin 31, a journal 52 parallel to the crank pin 31 and having a symmetrical center C3 deviated therefrom, and a first sprocket 53 cocentric to the crank pin 31. A bearing 42 provided at one end of a connecting rod 4 is matched rotatably to the journal 52. Further, a second sprocket 6 is rotatably arranged coaxially relative to the crank shaft 3, and the number of teeth 61 thereof is made to be twice that of the first sprocket 53. The first sprocket 53 is in mesh from the inside with a ring-shaped chain 65 wound around the second sprocket 6 and fixed thereto.