In the present study, we proposed the design formula of chord members of X-type tubular truss beams with branch joints. This formula is derived from the modified lateral buckling conditional equation obtained by the energy method, taking the constrained effect of rigid joints at crosing points of bracing members obtained by the results of the eigenvalue analyses into consideration. And also, we clarified quantitatively this effect to the lateral buckling behavior by the resuluts of the experiments and the non-linear analyses. Cosequently, we confirmed validity of the used assumptions and the proposed design formula.