To dissolve a problem where since, in conversion between rotation motion by a general crank device and reciprocating motion, an operation mode of reciprocating motion forms sinusoidal motion, and the stroke and the cycle time of each process are specified to be the same, an optimum motion mode originally demanded for each process can not be set.

A planetary gear mechanism is used at a joint part between a crank (1) and a connecting rod (2). By adding motion of an arm (5) to circular motion of the crank, the effective length of the crank is periodically changed. This constitution makes the reciprocating motion mode of a piston non- sinusoidal. By setting a parameter on a mechanism, such as a ratio of the number of teeth, a motion mode is selectively set. A demand for reciprocating motion of a crank device other than an engine is various and by setting an optimum reciprocating motion mode according to demand characteristics, the selection range of design is widened and optimum design is practicable.