Pillar structure for vehicle body rear portion

A pillar structure for a vehicle body rear portion in which a slide door that opens and closes an opening portion provided in a vehicle side surface is provided, a pillar portion on a vehicle body rear portion side of the opening portion is composed of a closed section of an inner panel and an outer panel, and a fixing mechanism that fixes a rear portion of said slide door at a closed position of said slide door is provided at said pillar portion on the vehicle body rear portion side, wherein said fixing mechanism is provided on a surface facing a vehicle body front side of said pillar portion on the vehicle body rear portion side, a pillar reinforcement body that forms a quadrangular closed section in horizontal section is internally mounted to an inner surface of said pillar portion on the vehicle body rear portion side to which said fixing mechanism is mounted, and an interior-side reinforcement member is disposed between said pillar reinforcement body and said inner panel so as to have a gap in a vehicle width direction between the interior-side reinforcement member and said pillar reinforcement body.