PROBLEM TO BE SOLVED: To solve the problem that the cost of an apparatus for the temperature control of heat source water in a protected horticulture is raised because an energy-saving control that is the water temperature control of circulating water which is the heat source, and environmental control that is the air conditioning of the greenhouse such as the conditioning of room temperature and humidity in the greenhouse can not be separated in the conventional temperature control of the heat source water. SOLUTION: This method comprises controlling the temperature of the heat source water by a control part 12 only based on an amount of solar radiation detected by a solar radiation sensor 22 and an outside air temperature detected by an outside air temperature sensor 23 to separate the energy-saving control, that is, the water temperature control of circulating water, and the environmental control, that is, the air conditioning of the greenhouse in the greenhouse, and to achieve the reduction of the cost for the apparatus when heating the green house 10 by feeding the circulating heat source water through a three-way valve 12, and a going pipe 14a to, a radiating pipe 14b in, and a returning pipe 14c from the greenhouse 10 to radiate the heat.
