PROBLEM TO BE SOLVED: To provide an air conditioning system of a greenhouse capable of adjusting temperature, humidity and concentration of carbon dioxide inside a greenhouse in a level which is needed for growing plants.SOLUTION: A system comprises: piping 3 capable of forming a first air passage A leading outside the greenhouse via a first underground pipe 13 laid in the underground under a greenhouse 2 from an upper part inside the greenhouse, a second air passage B leading inside the greenhouse via a second underground pipe 18 laid in the underground under the greenhouse from outside the greenhouse, and a third air passage C leading back inside the greenhouse via the first underground pipe from inside the greenhouse； air blowers 8 and 9 for blowing air to the air passages； a temperature sensor 4 for detecting the temperature inside the greenhouse； and air passage control means 11. The air passage control means 11 allows air to flow only in the first and the second air passages A and B among the first to the third air passages, when the temperature inside the greenhouse is 25°C or higher, and allows air to flow only in the third air passage C among the first to the third air passages, when the temperature inside the greenhouse is 10°C or lower.【課題】温室内の温度、湿度および炭酸ガス濃度を作物の育成に必要なレベルで調整可能な温室の空調システムを提供すること。【解決手段】温室２内の上部から温室下の地中に埋設された第１地中管１３を経由して温室外に至る第１空気流路Ａと、温室外から温室下の地中に埋設された第２地中管１８を経由して温室内に至る第２空気流路Ｂと、温室内から第１地中管を経由して温室内に戻る第３空気流路Ｃと、を形成可能な配管類３と、前記空気流路に空気を流すための送風機８，９と、温室内の温度を検出する温度センサ４と、流路制御手段１１と、を備える。流路制御手段１１は、温室内の温度が２５℃以上のとき、第１～第３空気流路のうち、第１および第２空気流路Ａ，Ｂのみに空気を流し、温室内の温度が１０℃以下のとき、第１～第３空気流路のうち、第３空気流路Ｃのみに空気を流す。【選択図】図４
