A vehicle body reinforcing structure includes: a panel-shaped reinforcing member having a through-hole penetrating through the reinforcing member in a plate-thickness direction of the reinforcing member and located inside a closed-section structure of a vehicle body framework portion; a first panel located at a position closer to an outside of the vehicle than the reinforcing member is; a second panel constituting the closed-section structure with the first panel and located at a position closer to an inside of the vehicle than the reinforcing member is; and a foaming adhesive member filled in an internal space of the closed-section structure, wherein the first panel and the second panel are bonded to each other by the foaming adhesive member penetrating through the through-hole of the reinforcing member, and the foaming adhesive member is filled between the first panel and the reinforcing member and between the second panel and the reinforcing member.