To suppress increases in manufacturing costs and vehicle weight by a simple construction in the understructure of a truck loading space structure of a truck loading space.

The understructure of the loading space is mounted on a pair of chassis frames 2a and 2b of the truck through a plurality of cross sills 3 respectively disposed in the vehicle width direction. The understructure of the loading space includes: a floor plate section 4 for forming a floor section of the loading space; and a support member 5 which is bridged in the vehicle width direction ahead of the plurality of cross sills 3 on the pair of chassis frames 2a and 2b, and supports a front end portion 4a of the lower surface of the floor plate section 4. A lower part 5f of the support member 5 between the chassis frames 2a and 2b is curved upward.