A vehicle body floor structure is provided with a floor, a side sill extendedly arranged at a side of the floor and in a longitudinal direction of a vehicle body to have a closed section, a pair of floor cross members extendedly arranged on the floor in a widthwise direction of the vehicle body, so as to face with each other in the longitudinal direction of the vehicle body, and a first reinforcing member, provided between the pair of floor cross members in the longitudinal direction of the vehicle body and within the closed section of the side sill, to exhibit property allowing a dimensional condition thereof to be maintained against a side collision load.