To provide an underfloor support member, in which height adjustment can be efficiently performed, and in which harmful insects such as bacteria and termites can be eliminated.

This underfloor support member 1 is installed between an underfloor member and an underfloor surface in a building for supporting and reinforcing the underfloor member. A support member main body 11 comprises a trunk part 2 having a female screw part, and support parts 3A and 3B having a base seat part 31 to be applied to the underfloor member or the underfloor surface, and a shaft part 32 having a male screw part to be engaged with the female screw part in the trunk part 2. A chemical agent container 12 is provided comprising a container main body 4 composed of a recessed part 43 to which chemical agent is inserted, and a plurality of cut parts 42, and a cover body 5 to cover the container main body 4. In the base seat part 31 of the support member main body 11, the container main body 4 of the chemical agent container 12 can be installed.