To improve sealability in a drain socket.SOLUTION: A drain socket connects an exhaust port of a flush toilet bowl with a drain pipe installed on a floor surface. The drain socket comprises a toilet bowl side connection part, a floor side connection part, a bent part and a seal member. The toilet bowl side connection part has an upper opening part to be connected with the exhaust port. The floor side connection part has a lower opening part to be connected with the drain pipe. The bent part is connected with the floor side connection part and is inserted in the toilet bowl side connection part. The seal member seals the toilet bowl side connection part and the bent part. The toilet bowl side connection part is provided with an insertion port. The bent part is provided with an insertion part to be inserted in the insertion port. At least one of the insertion port and the insertion part is provided with a step part. The seal member is installed in an air gap formed by the step part, the insertion part and the insertion port.SELECTED DRAWING: Figure 5