Plug-in adaptive controller (Plug-in AC) can reject the periodic disturbance in adaptive manner at selected frequencies independently, and its design method is based on external model principle which the controller for rejecting disturbance is placed outside the original feedback loop. In this paper, we propose Plug-in AC for discrete-time systems. Discrete-time Plug-in AC can be designed by using linear time invariant (LTI) representation of adaptive structure. We show more simple design method of such plug-in AC than the conventional design method. Our proposed method here is developed through the root locus of the poles on unit circle and can be developed by using less information than our conventional method.