Conventionally, various modes of rocker structures are known as a rocker structure of a vehicle body. For example, Patent Literature 1 described below discloses a side sill (rocker structure) configured by joining a sill inner composed of an extruded member and formed to have a rectangular cross-section (so-called an 8-shape cross section) so that a cross-sectional shape has an intermediate rib inside thereof with a sill outer composed of a press-molded member and formed in a C-cross-sectional shape.
Note that Patent Literature 2 described below discloses a front member of a front side frame that is integrally extrusion molded of a frame member composed of a pair of upper and lower square pipe members and having a closed cross-section shape and a flat plate portion for coupling the frame members. Further, Patent Literature 3 described below discloses a front side frame configured by joining two members disposed in a vehicle width direction in confrontation. The members have a convex portion in a central portion caused to project to the other member, an upper end flange portions, and a lower end flange portion, and the front side frame is configured by joining the upper end flange portions with each other and the lower end flange portions with each other, respectively. The front side frame has portions in which closed cross-sections are formed in an up-down direction by joining also the convex portions with each other, and the portions are used as buckling portions in a vehicle width direction with respect to a load in a vehicle body backward-forward direction. Further, Patent Literature 4 described below discloses a front side member that has a front end portion composed of two upper and lower closed cross-sectional structures having a cross-section formed in an 8-shape and has a portion succeeding the front end portion composed of a single closed cross-sectional structure having a rectangular cross-section so that rigidity is gradually increased from forward to backward. Further, Patent Literature 5 described below discloses a side sill front end portion coupling structure for coupling a side sill front end portion with a lower portion of a front pillar, wherein a load input in a vehicle body backward-forward direction is absorbed by deforming a lower portion of the front pillar.