To provide a method of adjusting temperature of heat source water for heating a horticultural greenhouse which prevents, without depending on heat dissipation by, ventilation or the like, excessive temperature increase in the greenhouse resulting from solar radiation after dawn and radiation of heat from the heat source water whose temperature increases to 70 to 80°C in the heat radiation pipe.

This method is to control temperature of heat source water for a horticultural greenhouse, equipped with a 3-way valve 12, circulation pump 13 and water temperature sensor 15 in the piping system 14 extending in the greenhouse to circulate heat source water from a boiler 11 and returns it back to the boiler 11, and a 3-way valve controlling section 12a, water temperature sensor 15, solar radiation sensor 22 and ambient temperature sensor 23, wherein temperature of the circulating heat source water is corrected and controlled, based on solar radiation and difference between ambient temperature and set point for controlling air temperature in the greenhouse.