To provide a drainage structure capable of enhancing a working efficiency in a bathtub pan and of preventing reversely flowing water from leaking out.

A groove part 11 opened from the bottom 7 of the bathtub pan 3 upward projects downward, and an opening 16 for operation is formed through the groove part 11. The draining structure includes a lid 20 which is attached to the opening 16 for operation through a sealant 21 and closes the opening 16 for operation. A drain passage insertion port 24 is formed in the lid 20. A drain passage 13 one end of which is connected to the drain port 12 of the bathtub 2 supported by the bathtub pan 3 is stored in the groove part 11. The other end of the drain passage 13 is extracted from the bathtub pan 3 through the drain passage insertion port 24 of the lid 20 attached to the opening part 16 for operation.