PURPOSE: To steplessly vary the phase between two synchronous shafts during operation by providing a reciprocation source mechanism on each of both synchronous shafts, supporting both synchronous shafts in parallel to each other on a machine frame and fixing sprocket gears having the same number of teeth with which a common endless chain is engaged to both synchronous shafts respectively.

CONSTITUTION: The operating phase of the cam 22 of a second synchronous shaft 20 is advanced with respect to the operating phase of the cam 12 of a first synchronous shaft 10, with the moving direction of an endless chain 30 between the sprocket gears 13, 23 of the two synchronous shafts 10, 20 as a reference. A slider 41 is moved in the direction of expanding or increasing the chain 30 of the loop portion (33a or 33b) in the portion where the shaft 10 side is positioned on the lower course of the shaft 20 side while reducing or contracting the chain of the loop portion on the other side, by rotating a feed screw. Thereupon, the chain 30 of the portion engaged with a gear 23 is moved in the rotating direction in line with the moving quantity of a slider 41 to draw in the chain 30 on the upper course side position of the gear 23 into the gear 23, advancing the phase of the shaft 20 with respect to the shaft 10.