[Problems to be Solved]In the valve opening and closing timing controlled by an ECU, since the responsivity of a phase varying apparatus becomes low and the valve opening and closing timing cannot be appropriately controlled in the state of low engine rpm, desired engine characteristics cannot be obtained. Therefore, it is a problem to be solved by the present invention to provide a method and an apparatus for controlling a phase varying apparatus capable of obtaining the considerably high responsiveness of the phase varying apparatus under condition of low engine rpm.[Solution]The above problem can be solved by a phase control method for controlling a phase varying apparatus constituted so as to calculate an estimated relative angle value one step later by the Kalman filter using a relative angle calculated from rotational velocity values of the associated shafts in the current step and a command value to an electromagnetic clutch in the current step, input the estimated relative angle value to a sliding mode controller as a feedback signal, calculate a command value to the electromagnetic clutch one step later based on the value of the feedback signal and output the command signal.