To realize a boundary beam damper capable of rationally controlling vibration in a steel framed structure without interfering with a space in a building.

The boundary beam damper 1 is constituted of a pair of H- shaped steels 2 arranged between columns 3 of the building in parallel, one ends 2d of them are fastened to the sides of the columns 3 in advance by welding, the other ends 2e are connected to each other with a fastening means such as a bolt 5 or the like through a splice plate 4 to form a boundary beam of a short span erecting over between the columns 3. The H-shaped steels 2 constituting the boundary beam damper 1 are so constituted that upper flange 2a as an upper member and a web 2b are made of a high tension steel and that a lower flange 2c as a lower member is made of a different member such as an extreme soft steel with low yield point. An RC slab 6 can be directly provided to the upper flange 2a of the boundary beam damper 1 constituted by such a method, and the boundary beam damper 1 and the RC slab 6 are integrally constituted.