A side vehicle body structure comprising: left and right rear wheel houses extending upward from vehicle-widthwise outer side portions of a floor section constituting a rear floor of a vehicle body; upper stiffeners joined to corresponding ones of the rear wheel houses and protruding toward a middle of a passenger compartment; and pillars extending upward from the corresponding ones of the rear wheel houses and each including an inner pillar member protruding toward the middle of the passenger compartment, each of the upper stiffeners having an extension section extending upward from the corresponding rear wheel house to be superposed on a lower end of the inner pillar member so that the upper stiffener connects to the inner pillar member via the extension section, wherein each of the rear wheel houses has a first closed-sectional portion formed by the upper stiffener being joined thereto, each of the pillars includes a pillar stiffener closing an opening of the inner pillar member by being joined to an outside of a passenger compartment in opposed relation to the inner pillar member, and a second closed-sectional portion defined by the inner pillar member and the pillar stiffener and extending in an up-down direction of the side vehicle body structure, and the pillar stiffener includes a lower-end extension extending downward beyond the lower end of the inner pillar member, the lower-end extension being located in overlapping relation to the extension section of the upper stiffener, wherein each of the rear wheel houses includes: an inner rear wheel house member disposed proximate to the passenger compartment; and an outer rear wheel house member disposed outside of the passenger compartment and joined to an end portion of the inner rear wheel house member; the upper stiffener is disposed on a surface of the inner rear wheel house member facing toward the passenger compartment, and a lower end of the lower-end extension of the pillar stiffener is joined to an upper surface of the outer rear wheel house member, characterized in that each of the rear wheel houses includes a lower stiffener joined from below to the inner rear wheel house member and the outer rear wheel house member in superposed relation to the inner rear wheel house member and the outer rear wheel house member, the lower stiffener overlaps the upper stiffener and the pillar and has a vehicle-widthwise outer side portion that extends to a joint portion of the outer rear wheel house member having the pillar stiffener joined thereto, and the inner pillar member has a reinforcing member for reinforcing the lower end thereof on which the extension section of the upper stiffener is superposed, the reinforcing member, the extension section of the upper stiffener and the lower end of the inner pillar member being overlapped on one another and joined together. left and right rear wheel houses extending upward from vehicle-widthwise outer side portions of a floor section constituting a rear floor of a vehicle body; upper stiffeners joined to corresponding ones of the rear wheel houses and protruding toward a middle of a passenger compartment; and pillars extending upward from the corresponding ones of the rear wheel houses and each including an inner pillar member protruding toward the middle of the passenger compartment, each of the upper stiffeners having an extension section extending upward from the corresponding rear wheel house to be superposed on a lower end of the inner pillar member so that the upper stiffener connects to the inner pillar member via the extension section, wherein each of the rear wheel houses has a first closed-sectional portion formed by the upper stiffener being joined thereto, each of the pillars includes a pillar stiffener closing an opening of the inner pillar member by being joined to an outside of a passenger compartment in opposed relation to the inner pillar member, and a second closed-sectional portion defined by the inner pillar member and the pillar stiffener and extending in an up-down direction of the side vehicle body structure, and the pillar stiffener includes a lower-end extension extending downward beyond the lower end of the inner pillar member, the lower-end extension being located in overlapping relation to the extension section of the upper stiffener, wherein each of the rear wheel houses includes: an inner rear wheel house member disposed proximate to the passenger compartment; and an outer rear wheel house member disposed outside of the passenger compartment and joined to an end portion of the inner rear wheel house member; the upper stiffener is disposed on a surface of the inner rear wheel house member facing toward the passenger compartment, and a lower end of the lower-end extension of the pillar stiffener is joined to an upper surface of the outer rear wheel house member, each of the rear wheel houses includes a lower stiffener joined from below to the inner rear wheel house member and the outer rear wheel house member in superposed relation to the inner rear wheel house member and the outer rear wheel house member, the lower stiffener overlaps the upper stiffener and the pillar and has a vehicle-widthwise outer side portion that extends to a joint portion of the outer rear wheel house member having the pillar stiffener joined thereto, and the inner pillar member has a reinforcing member for reinforcing the lower end thereof on which the extension section of the upper stiffener is superposed, the reinforcing member, the extension section of the upper stiffener and the lower end of the inner pillar member being overlapped on one another and joined together.