A temporary drainage system for draining rain water that flows into a floor under construction to an outside of a building during construction of a multiple-story building includes: a flexible drainage collection hose connected to a penetration hole formed in a floor slab of the floor under construction; a sand-settling vessel provided on a floor slab of a constructed floor of the building and connected to the drainage collection hose; a drainage pit provided on a lowest floor of the building to store drainage water discharged from the sand-settling vessel through a drainage hose; and a drainage pump provided in the drainage pit. The drainage water collected into the drainage pit is discharged to the outside of the building by an operation of the drainage pump.