An air-conditioning apparatus, comprising: a plurality of indoor units (120a, 120b, 120c); and an outdoor unit comprising: an outdoor heat exchanger for exchanging heat between refrigerant delivered from the plurality of indoor units (120a, 120b, 120c) 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 (120a, 120b, 120c), the plurality of indoor units (120a, 120b, 120c) each comprising an operating-state notifying unit 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 (120a, 120b, 120c) 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 (120a, 120b, 120c) performing the heating operation changes; and an operation control unit 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 (120a, 120b, 120c); and an outdoor unit comprising: an outdoor heat exchanger for exchanging heat between refrigerant delivered from the plurality of indoor units (120a, 120b, 120c) 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 (120a, 120b, 120c), an outdoor heat exchanger for exchanging heat between refrigerant delivered from the plurality of indoor units (120a, 120b, 120c) 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 (120a, 120b, 120c), the plurality of indoor units (120a, 120b, 120c) each comprising an operating-state notifying unit 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 (120a, 120b, 120c) 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 (120a, 120b, 120c) performing the heating operation changes; and an operation control unit 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 (120a, 120b, 120c) 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 (120a, 120b, 120c) performing the heating operation changes; and an operation control unit 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.