A clutch actuator relatively moves a first clutch member and a second clutch member relative to each other in an axial direction. A phase difference sensor detects a phase difference between the first and second clutch members. A controller executes a synchronization operation that gradually decreases an input-output rotational speed difference until it reaches a target rotational speed difference and detects a current latest engagement timing based on an output of the phase difference sensor. After an adjustment reference time point, at which the input-output rotational speed difference reaches a phase-difference-detectable rotational speed difference that allows the phase difference sensor to detect the phase difference, the controller adjusts the phase difference by changing a change characteristic of the input-output rotational speed difference such that one of future engagement timings coincides with a target completion time point, based on information obtained at an arbitrary phase difference detection time point.