This study is on the bond characteristic between a flat bar and a concrete in the expansive concrete filled steel tube with the flat bar in center. This tube is used for the brace member of the energy dissipative braced frame. To obtain the bond characteristic, I carried out the repeated tension and compression loading test of it. And then I obtained the relationship between the bond stress and the slip on the interface of them under tension load. Based on this relationship, by solving the differential equation on this bond characterisic, I found the possibilty of the evalution of this tube's stiffness.