To provide a boundary beam capable of rationally restricting vibration in an RC structure without prohibiting space in a building.

In this boundary beam 1, a plurality of X-shaped bar arrangements 2 each comprising two flat bars 3 combined with each other in an X-form are disposed in parallel in a beam width direction. The X-shaped bar arrangement is disposed in such a way that an intersection of the two flat bars 3 combined in the X-form is positioned to a center point A in a cross section as the boundary beam 1 is seen in a horizontal direction. Its end part is contained in an adjacent RC core wall 7 on which the boundary beam 1 is stretched, and the X-shaped bar arrangement is rigidly coupled with the RC core wall 7. On an end part of the flat bar 3, a stud dowel 8 is preliminarily mounted to improve fixing strength to concrete 6 composing the RC core wall 7.