In a vehicle floor frame structure, side members 2 each include a floor side member 21 and a rear side member 22; the floor side member 21 is disposed on a main floor panel 11; the rear side member 22 is disposed on a rear floor panel 12; a reinforcement 3 is stacked on the floor side member 21 in a vertical direction with the main floor panel 11 interposed therebetween; an upper surface 3a of the reinforcement 3 becomes higher toward the rear of the vehicle body; a front portion 3b and lower portions of side surfaces 3c of the reinforcement 3 are joined to the main floor panel 11; a rear end portion 3d of the reinforcement 3 is joined to the vertical wall 13a in a front portion of the rear floor panel 12; a closed section space S1 enclosed by the main floor panel 11, a front portion 13 of the rear floor panel 12, and the reinforcement 3 is formed in a side view; a lower surface portion 21b of a rear end of the floor side member 21 becomes higher toward the main floor panel 11; and the upper surface 3a of the reinforcement 3 and the lower surface portion 21b of the floor side member 21 are inclined at almost the same angles and are formed to have a monotonous cross section in a side view.