PURPOSE: To disuse the conventional float so as to improve durability by opening a pilot switching valve of a drain pipe when a difference in pressure between output ports determined by water levels of both large and small area chambers in plural drain containers reaches preset pressure.

CONSTITUTION: Air supplied from an air pressure source 18 to the primary side pipe 5 branches off, flows into plural drain containers 1, 2 through the respective inlet ports 4, 9, and bypasses the respective partition plates 3, 8 to be discharged from the respective outlet ports 6, 10 to the secondary side pipe 7. Drain contained in the air is separated and removed by an air filter or the like to be stored in the bottoms of both drain containers 1, 2. When a pressure difference between the pressure of the outlet port 6 determined by the water level in the small area chamber A in one drain container 1 and the pressure of the outlet port 6 determined by the water level in the large area chamber B in the other drain container 2 reaches a preset pressure, a pilot switching valve 13 of a drain pipe 12 is opened to discharge the stored drain.