PURPOSE: To reduce the size and the cost of a device, by a method wherein a first trunnion for supporting a first power roller and a second trunnion for supporting a second power roller are disposed rotatably around their respective axes and reciprocatively movably along the axes.

CONSTITUTION: Through rotation of a control shaft 133, first and second trunnions 122 and 123 are displaced along their respective axes, and displacement directions are extended symmetrically to each other. In this case, since the control shaft 133 is situated between the trunnions 122 and 123, there is no need for a specially large space for control. Besides, since first and second cam grooves 131 and 132 are formed in a manner to be inclined, along with rotation of the control shaft 133, even when the trunnions 122 and 123 are displaced in their respective axis directions, along with the displacement, first and second power rollers 120 and 121 vary their respective rotation angle, the two trunnions 122 and 123 are rotated around their respective axes, and are returned to a neutral position again.