This vehicle body side structure includes: side sills provided at both end portions of a floor section in a width direction of a vehicle and extending in a forward/rearward direction of the vehicle; a pillar extending upward from each of the side sills and having a pillar outer provided on an outside in the width direction of the vehicle; a crushing member provided on an inside in the width direction of the vehicle with respect to the pillar outer, having a lower portion joined to the side sill, and extending upward; an impactor which is provided above the crushing member, which is joined to an inside of the pillar outer in the width direction of the vehicle, and in which the impactor and the crushing member at least partially overlap in an upward/downward direction; and a seatbelt winding device provided on the outside in the width direction of the vehicle with respect to the crushing member and disposed inside an accommodation concave section formed in the impactor.