The problem of suppressing the tone noise due to interaction of the rotor blades with the gust by means of the actuator motion on the duct surface is studied. Theoretical analysis with numerical calculations is conducted for a simple harmonic sinusoidal gust and a simple harmonic sinusoidal circumferential wave form of the actuator motion, and optimum conditions of the actuator motion are investigated. The results are summarized as follows: 1) If the noise field is composed of a single cut-on duct mode only, complete suppression of either upstream acoustic power or downstream acoustic power is possible. 2) Simultaneous suppression of upstream and downstream acoustic powers is in general difficult. 3) In the case of multiple cut-on duct modes, complete suppression of noise is unattainable. 4) The modal structure of the sound waves from the actuator surface on the duct wall is substantially different from those of the sound waves generated from rotor blades. 5) In the case ofthe primary noise ofa cut-on circumferential wave number with multiple cut-on radial modes, an actuator motion generating sound waves of the cut-on circumferential wave number cannot effectively suppress the noise because the cancellation of the original gust-rotor interaction noise by the direct secondary sound from the actuator surface is essentially unsuccessful. 6) In the case of multiple cut-on radial modes, actuator motions of a cut-off circumferential wave number can successfully suppress the sound level, provided that the actuator motion of a large amplitude is possible. In this case, the original primary noise of cut-on modes due to rotor-gust interaction is subtracted only by the blade-interfering secondary noise of cut-on radial modes due to interaction of the rotor blades with the direct secondary disturbances from the actuator, whereas the direct secondary disturbances, which are cut off, decay exponentially as they propagate upstream and downstream.