PURPOSE: To regulate the length of rhizomic parts according to a level of underground water surface by providing a specific water feed and discharge member under a growth soil layer of rhizomic parts under a compost layer and producing underground water to a prescribed depth in the growth soil layer of the rhizomic parts.

CONSTITUTION: A water feed and discharge pipe 4 is arranged in a position at a prescribed depth of a cultivation layer 1 provided with a growth soil layer 3 of rhizomic parts at a prescribed depth from a compose layer 2 and lateral parts of the water feed and discharge pipe 4 are raised up toward the ground surface. Water is fed from a water feed and discharge device 6 to the water feed and discharge pipe and delivered from plural holes formed on the surface of the water feed and discharge pipe. Thereby, the underground layer level (A-A) is kept constant at a prescribed depth of the soil layer 3.