A control input for controlling a driving force of an actuator (5), by a control processing of the sliding-mode control using a switching function configured from a first variable component, which is a deviation between an observed value of a secondary power imparted to a secondary element (3) from a primary element (2) via an elastic deformation member (4), and a second variable component which is a temporal change rate of the deviation, so as to converge the first variable component to zero on the switching hyperplane. A gradient of the switching hyperplane is preliminarily determined based on a response characteristics data satisfying a predetermined requirement.