PURPOSE: To prevent a device from being damaged owing to drying and cooling, by providing the upstream end of an underdrainage pipe with a service pipe, and by providing the downstream end with a siphon drain pipe.

CONSTITUTION: The upstream end of an underdrainage pipe 2 buried in cultivation bed 1 is provided with a service pipe 3. A siphon drain pipe 5 is made to branch off on the way from the underdrainage pipe 2 to a drain ditch 9, and the drain ditch 9 is provided with a drain stop plug 4. Warm water flowing into the underdrainage pipe 2 is discharged from a water-discharging hole 7 and is heat-exchanged with the cold temperature of the cultivation bed 1, and cold air is discharged from the cultivation bed 1 with water to increase temperature of the cultivation land.