Some approaches to the simultaneous optimum design of multiple dynamic absorbers (DAs) are presented for suppressing multiple resonance peaks of a multiple-degrees-of-freedom (MDOF) structure simultaneously, taking changes of natural modes and natural frequencies of the structure due to added mass effects of DAs into account in the design. First, some methods for determining an equivalent single-degree-of freedom (SDOF) system of the subject MDOF structure are explained, in order to make possible the application of the usual design method for the SDOF system by Den Hartog to the subject MDOF structure. Second, some guides in determining the best place on which DAs should be attached in the subject structure are indicated for giving the best play to DAs efficiencies. Third, three kinds of approaches to the simultaneous optimum design are proposed using the modal analysis. Validity and usefulness of the proposed approaches are demonstrated by both computer simultation and experiment using a plate and a simple structure.