To provide building indoor underfloor piping and a work execution method of underfloor piping easily executable at a low cost.

Drain piping disposed under the floor of an indoor living space is composed of a catch basin 1, a main drain pipe 7 connected to a drain port of the catch basin 1, and branch drain pipes 8 connecting the drain port of the catch basin 1 to drain equipment D. The branch drain pipes 8 are formed of flexible pipes, and each flexible pipe is composed of an inner pipe with a smooth inner surface formed of flexible resin, and a protective pipe formed of rigid resin. The catch basin 1 is provided with an air intake valve. In this work execution method of indoor underfloor piping using the catch basin 1, the branch drain pipes 8 are previously cut in appropriate length in a factory or the like, connected to the drain port of the catch basin 1 and then carried into a construction site for piping.