PROBLEM TO BE SOLVED: To solve the problem unsuitable for a crop that a controlling temperature in a greenhouse to be kept is changed by about +2 deg.C because the room temperature in the greenhouse is raised to keep the relative humidity in the greenhouse for the protected horticulture at the value at which dewfall is prevented. SOLUTION: This method comprising mixing boiler water in a boiler 11 with returning water returned from a returning pipe 14c by a three-way valve 12 to control the water temperature of a heat source for heating so that dewfall may be prevented without changing the controlling temperature of the greenhouse, comprises sending a command to the three-way valve 12 so as to keep the water temperature of the heat source at about 40 deg.C of the water temperature hardly raising the room temperature unnecessarily when the relative humidity in the greenhouse 10 is high, and intermittently on-off controlling a circulating pump 13 for sending the water for the heat source to a radiation pipe 14b by a controlling part 21 receiving information from a room temperature sensor 22 and a humidity sensor 23.
