An air-conditioning apparatus , comprising: a plurality of indoor units ; and an outdoor unit comprising: an outdoor heat exchanger for exchanging heat between refrigerant delivered from the plurality of indoor units and outdoor air; and a compressor for compressing the refrigerant output from the outdoor heat exchanger so as to deliver the compressed refrigerant to the plurality of indoor units , the plurality of indoor units each comprising an operating-state notifying unit (16-2) for transmitting operating-state information indicating that a heating operation is being performed to the outdoor unit , characterised in that the outdoor unit comprises: an outdoor-unit control device including a frost-formation determining unit (17-2, 17-3, 17-4) configured to determine a number of the indoor units performing the heating operation based on the operating-state information and determine whether there is occurrence of frost formation on the outdoor heat exchanger after elapse of a preset time period from a time at which the number of the indoor units performing the heating operation changes; and an operation control unit (17-5) configured to perform a defrosting operation when the frost-formation determining unit (17-2, 17-3, 17-4) determines there is the occurrence of the frost formation. a plurality of indoor units ; and an outdoor unit comprising: an outdoor heat exchanger for exchanging heat between refrigerant delivered from the plurality of indoor units and outdoor air; and a compressor for compressing the refrigerant output from the outdoor heat exchanger so as to deliver the compressed refrigerant to the plurality of indoor units , an outdoor heat exchanger for exchanging heat between refrigerant delivered from the plurality of indoor units and outdoor air; and a compressor for compressing the refrigerant output from the outdoor heat exchanger so as to deliver the compressed refrigerant to the plurality of indoor units , the plurality of indoor units each comprising an operating-state notifying unit (16-2) for transmitting operating-state information indicating that a heating operation is being performed to the outdoor unit , characterised in that the outdoor unit comprises: an outdoor-unit control device including a frost-formation determining unit (17-2, 17-3, 17-4) configured to determine a number of the indoor units performing the heating operation based on the operating-state information and determine whether there is occurrence of frost formation on the outdoor heat exchanger after elapse of a preset time period from a time at which the number of the indoor units performing the heating operation changes; and an operation control unit (17-5) configured to perform a defrosting operation when the frost-formation determining unit (17-2, 17-3, 17-4) determines there is the occurrence of the frost formation. an outdoor-unit control device including a frost-formation determining unit (17-2, 17-3, 17-4) configured to determine a number of the indoor units performing the heating operation based on the operating-state information and determine whether there is occurrence of frost formation on the outdoor heat exchanger after elapse of a preset time period from a time at which the number of the indoor units performing the heating operation changes; and an operation control unit (17-5) configured to perform a defrosting operation when the frost-formation determining unit (17-2, 17-3, 17-4) determines there is the occurrence of the frost formation. <EOS>