To provide a stroke variable device where a stroke can be easily made variable even under driving, and further, miniaturization and the reduction of cost are made possible even with a relatively simple constitution.

The stroke variable device is equipped with: a first eccentric shaft 32 provided at a first main rotary body 20; a second eccentric shaft 33 provided at a second main rotary body 21; a ram 19 supported in a state of being linearly movable back and forth; a transfer mechanism 35 of transferring the displacement of the first eccentric shaft 32 and the second eccentric shaft 33 in the upper and lower directions to one end side of the ram 19; a stroke setting means of setting the stroke of the ram 19; a phase difference deciding means of deciding the phase difference between the first eccentric shaft 32 and the second eccentric shaft 33; a phase difference adjustment controlling means of making the phase difference between the first eccentric shaft 32 and the second eccentric shaft 33 coincident to the decided phase difference; and a reciprocation controlling means of rotating the first main rotary body 20 and the second main rotary body 21 at the same speed and reciprocating the ram 19.