A new optimal damper design method, called 'consecutive design generation method (CDGM)', is presented for elastic-plastic multi-degree-of-freedom (MDOF) building structures under multiple-level earthquake ground motions (GMs). The procedure of the proposed method is as follows: (i) Set a variety of levels of input GMs {IM1,..., IML} (IM1 <... < IML). (ii) Obtain the optimal design under GM with IM = IM1 using an optimization method. (iii) Obtain the optimal design under GM with IM = IM2 by a local search around the optimal design under GM with IM = IM1 as the initial design. (iv) Obtain the optimal design under GM with IM = IM3 by a local search around the optimal design under GM with IM = IM2 as the initial design, and the same procedure is repeated under GMs with other IMs. By using the proposed method, all the optimal designs can be obtained with much lower computational effort than the method repeating an ordinary optimization procedure under every-level GM. The proposed method helps to understand how the distribution of added damper damping coefficients changes as the level of GMs changes. Furthermore, a simple design guideline for the problem, which design to be chosen from various optimal designs, is presented based on the 'ideal drift response curve', which is also a new concept. Two representative recorded ground motions are employed as a set of GMs and the proposed method is applied to these GMs. The influence of the number of GMs and the level of GMs on the optimal damper design is clarified.