PURPOSE: To accumulate solar heat without hindering photosynthesis of plants and contrive to enhance an energy-conserving effect, by a method wherein a wall form heat collector comprising a heat-collecting or heat-releasing passage is integrated with a greenhouse by combining a heat-absorbing transparent plate and a transparent plate with each other, and the assembly is thermofluidically connected to an underground heat exchanger.

CONSTITUTION: A heat-absorbing transparent plate 5a and a transparent plate 6b are faced to each other to form the heat-collecting or heat-releasing passage 7, and transparent plates 6a, 6c are faced to the outside of the plates 5a, 6b to form heat-insulating closed space layers 8a, 8b therebetween. When the temperature of a heat collector 2 is high, a blower 4 is operated to accumulate heat underground, and when room temperature is high, room air is passed into the heat collector 2 to effectively collect surplus heat in the greenhouse 1. Heat- insulating characteristic of the heat collector 2 at night is secured by at least double curtains, and warm air is circulated from the underground heat exchanger 3, so that a synergistic warming effect is obtained.