A structure of seat back in vehicle seat, wherein a back board element is provided the said backward side of the seat back in such a manner as to normally provide a substantially flat plane rigid enough to withstand one load applied toward a backward side of the back board element, thereby to allow the back board element to be usable as a means for carrying load and cargo thereon, while being resiliently deformable in a backward direction of the vehicle seat by another load applied toward a seat occupant's back support side of said seat back. A plurality of crevices may be defined properly in one side of the back board element so as to effectively permit the back board element to be either in the substantially flat state or in a resiliently deformable state.