A controller for a motor cleans, as necessary, a hydraulic chamber that generates hydraulic oil for changing a phase difference between two rotors so as to restrain sludge from building up in the hydraulic chamber. A motor has a first rotor and a second rotor that can be relatively rotated with respect to the first rotor. The controller includes a phase difference changing driver that changes the phase difference between the two rotors by controlling the pressure in the hydraulic chamber filled with hydraulic oil. The controller further includes a cleaning need determiner that determines the need for cleaning the hydraulic chamber, and a cleaning phase difference controller that controls the phase difference changing driver such that the second rotor is relatively rotated in the forward direction and the reverse direction alternately with respect to the first rotor if a determination result of the cleaning need determiner is affirmative.