The underground water pipe system for energy-saving control of temperature of nutrient solution in the soil-less culture was newly proposed, and its performance was examined in the soil-less culture of Welsh onion (Allium fistulosum L. cv. Koutou). The system consists of a large-sized pipe filled with water under the ground (the underground water pipe), and a unit for circulating the nutrient solution between the cultivation bed and the underground water pipe. The temperature condition in the underground water pipe 1.5 m below the ground surface is stable as compared with that in greenhouses which excessively high in summer and low in winter. During the circulation, the heat can be exchanged between the nutrient solution and the water stored in the underground water pipe. The excessive heat loaded in the nutrient solution in the greenhouse during the hot sunny daytime was effectively removed into the water stored in the underground water pipe, and this enabled the normal growth of plants without serious stress by extremely high temperature in summer. Furthermore, this circulation warmed the nutrient solution excessively chilled in cold winter nights. Therefore, the underground water pipe system proposed is applicable to the energy-saving control of temperature of the nutrient solution in the soil-less culture for year round production of vegetables.