An air conditioning system in which a single heat source unit (A) comprising a compressor (1), a four-way valve (2), a heat source unit side heat exchanger (3) and an accumulator (4) is connected to a plurality of indoor units (B, C, D) through a first and a second connection pipes (6, 7). Each indoor unit (B, C, D) comprises a suction air temperature detecting device (50) for detecting a suction air temperature of the indoor unit, an opening degree setting device for setting a minimum valve opening degree of the first flow rate controller 9 in accordance with the difference between the target temperature and the suction air temperature and a first valve opening degree control device (52) for controlling the valve opening degree of the first flow rate controller 9 at a predetermined rate to the minimum valve opening degree. A bypass system is further provided to enable an effective defrost operation.