Construction of a side impact bar assembly. When side impact is not occurred, a stopper pin (32) is engaged into pores (40 and 42) to securely hold an auxiliary bar (14) to a side impact bar (12). If side impact is occurred, as an impact plate (26) is pushed, the stopper pin (32) is disengaged from the pores (40 and 42). Then, the auxiliary bar (14) projects outward through the end of the side impact bar (12) by biasing force of a spring (24), and the projecting end of the auxiliary bar (14) is inserted into a center pillar outer panel (22) through holes (34, 36 and 38).