We proposed the design concept for steel shear panel damper. We introduced the design requirements for width-to-thickness ratio of shear panel damper's flange. To show the validity of the design requirements, we performed the cyclic loading test with incremental intensity and geometrically and material non-linear finite element analyses on shear panel dampers. From those results, we discussed the validity of the proposed design requirements for S.P.D.'s flange width-to-thickness ratio.