A vehicle side body structure including: a side sill that has a substantially closed cross-sectional structure substantially extending in a vehicle longitudinal direction; and a center pillar that has a substantially closed cross-sectional structure substantially extending in an upward direction from said side sill, wherein an opening (2r, 2f) is formed at a position above the side sill and at each of front and rear sides of the center pillar, wherein the side sill includes a reinforcing member configured to reinforce bending rigidity against a load to an inner side in a substantially vehicle width direction, and the reinforcing member is partially provided in a cross-sectional view that is substantially perpendicular to a longitudinal direction of the side sill, and wherein the reinforcing member is configured to substantially extend along the longitudinal direction of the side sill such that at least a part of the reinforcing member overlaps a center pillar joined portion, to which the center pillar is joined, in a vehicle side view at a height position of said center pillar joined portion, the vehicle side body structure further comprising: a plurality of crossmembers (110, 120, 130) substantially extending to the inner side in the vehicle width direction from the side sill and separated from each other in the vehicle longitudinal direction, characterized in that: the vehicle side body structure further comprising, in a closed cross-sectional space on the inside of the side sill, a section (21F, 21R, 22F, 22R) that is configured to at least partially partition said closed cross-sectional space in a substantially longitudinal direction at least in an upper portion corresponding to a height of the center pillar joined portion. the side sill includes a reinforcing member configured to reinforce bending rigidity against a load to an inner side in a substantially vehicle width direction, and the reinforcing member is partially provided in a cross-sectional view that is substantially perpendicular to a longitudinal direction of the side sill, and wherein the reinforcing member is configured to substantially extend along the longitudinal direction of the side sill such that at least a part of the reinforcing member overlaps a center pillar joined portion, to which the center pillar is joined, in a vehicle side view at a height position of said center pillar joined portion, the vehicle side body structure further comprising: a plurality of crossmembers (110, 120, 130) substantially extending to the inner side in the vehicle width direction from the side sill and separated from each other in the vehicle longitudinal direction, characterized in that: the vehicle side body structure further comprising, in a closed cross-sectional space on the inside of the side sill, a section (21F, 21R, 22F, 22R) that is configured to at least partially partition said closed cross-sectional space in a substantially longitudinal direction at least in an upper portion corresponding to a height of the center pillar joined portion. a plurality of crossmembers (110, 120, 130) substantially extending to the inner side in the vehicle width direction from the side sill and separated from each other in the vehicle longitudinal direction, characterized in that: the vehicle side body structure further comprising, in a closed cross-sectional space on the inside of the side sill, a section (21F, 21R, 22F, 22R) that is configured to at least partially partition said closed cross-sectional space in a substantially longitudinal direction at least in an upper portion corresponding to a height of the center pillar joined portion.