To provide a method of regenerating a damaged drain trap which, when a water seal type drain trap is damaged, restores a trap function of the drain trap.

In a method of regenerating a damaged drain trap, the drain trap comprises a trap body 1 which is provided with a drain port 2 for receiving inflow of waste water and a discharge port 3 for discharging the waste water, and a water sealing member which is combined with the trap body 1 to form a water sealing part 4 and prevents a reverse flow from the downstream side of the drain port 2. The damaged drain trap is regenerated by removing the water sealing member and attaching a trap unit to the discharge port 3 or on a passage communicating with the discharge port 3.