The inner diameter at the bottom of a case is gradually decreased from upper to lower positions. A bottom surface of a communication hole of a drain housing provided in an opening of the case has an inclined surface inclined toward a drain discharge hole. An outer marginal portion which is positioned at the uppermost position in the bottom surface of the communication hole is in the same plane as or lower than a marginal portion which is positioned at the lowermost position in an inner circumferential surface of the case and adjacent to the opening.

A drain valve including a housing having a float chamber, a piston chamber, and a plenum communicating with the piston chamber through a seat. The housing has a liquid inlet passage, and an outlet port. The drain valve has a tube passage communicating with the piston chamber. The drain valve includes an annular float. The drain valve includes a seal body connected to the float and rising away from the tube passage when liquid is in the float chamber. The drain valve includes a piston slidably mounted in the piston chamber having a head that moves away from the seat when gas enters the piston chamber. The piston includes a valve element that moves as the piston head moves to expose the liquid inlet port and allow liquid to flow from the liquid inlet passage through the plenum and into the liquid outlet passage.