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 and a desired 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 sequentially determine a control input to control an actuator (5) to converge the first variable component to zero on a switching hyperplane. A gradient of the switching hyperplane is set such that a time constant corresponding to the gradient of the switching hyperplane is equal to or larger than a given specific time constant.