A method for predicting the maximum control force of a proportional- derivative controlled system is proposed based on a response spectrummethod. In designing structural control devices for the reduction of seismic response, it is important to choose an actuator with an adequate control force capacity in order to achieve a satisfactory control effect. To accomplish this, the maximum control force under assumed seismic ground motions has to be estimated.To verify the proposed method, the predicted maximum forces are compared with the results of a time-history analysis using white noise and design ground motions. With respect to the analysis models, three-story and 25-story building models with active bracing for each story are used. It is found that the differences between the predicted values of the maximum response and control force and the time-history analysis results are sufficiently small, and thus the proposed method is reliable to use in design.