A refrigerating and air-conditioning apparatus comprising: a plurality of refrigeration cycles that each include a compressor (3a, 3b), a four-way valve (4a, 4b), an outdoor heat exchanger (5a, 5b), a pressure reducing device (6a, 6b), and an indoor heat exchanger (7a, 7b) that are connected to one another, the plurality of refrigeration cycles being capable of independently performing a heating operation and a defrosting operation; an outdoor unit that includes the compressors (3a, 3b), the four-way valves (4a, 4b), and the outdoor heat exchangers (5a, 5b); a plurality of indoor units (2a, 2b) in each of which the indoor heat exchanger (7a, 7b) for respective one of the refrigeration cycle is disposed, and that each includes an indoor fan (8a, 8b) for taking in outside air through a ventilation port (9a, 9b) and blowing the air into a room, and a ventilation damper (10a, 10b) that opens and closes the ventilation port (9a, 9b); and a controller configured to control the ventilation damper (10a, 10b) of each of the indoor units (2a, 2b), wherein the plurality of indoor units (2a, 2b) being suitable to be installed in a same room, characterized in that the controller is configured to when at least one of the refrigeration cycles performs defrosting operation and another at least one of the refrigeration cycles performs the heating operation, render the ventilation damper (10a, 10b) of the indoor unit (2a, 2b) that performs the defrosting operation to be in a closed state during the defrosting operation, and control the ventilation damper (10a, 10b) of the indoor unit (2a, 2b) that performs the heating operation to achieve a required amount of ventilation corresponding to a ventilation state of the room, and in a case where at least one of the refrigeration cycles is stopped, when starting a defrosting operation by another refrigeration cycle, start the defrosting operation after activating the stopped refrigeration cycle to start a heating operation, cause the stopped refrigeration cycle to continue the heating operation even after the refrigeration cycle on the defrosting side that has started the defrosting operation terminates the defrosting operation, and stop the refrigeration cycle on the defrosting side after terminating the defrosting operation. a plurality of refrigeration cycles that each include a compressor (3a, 3b), a four-way valve (4a, 4b), an outdoor heat exchanger (5a, 5b), a pressure reducing device (6a, 6b), and an indoor heat exchanger (7a, 7b) that are connected to one another, the plurality of refrigeration cycles being capable of independently performing a heating operation and a defrosting operation; an outdoor unit that includes the compressors (3a, 3b), the four-way valves (4a, 4b), and the outdoor heat exchangers (5a, 5b); a plurality of indoor units (2a, 2b) in each of which the indoor heat exchanger (7a, 7b) for respective one of the refrigeration cycle is disposed, and that each includes an indoor fan (8a, 8b) for taking in outside air through a ventilation port (9a, 9b) and blowing the air into a room, and a ventilation damper (10a, 10b) that opens and closes the ventilation port (9a, 9b); and a controller configured to control the ventilation damper (10a, 10b) of each of the indoor units (2a, 2b), wherein the plurality of indoor units (2a, 2b) being suitable to be installed in a same room, characterized in that the controller is configured to when at least one of the refrigeration cycles performs defrosting operation and another at least one of the refrigeration cycles performs the heating operation, render the ventilation damper (10a, 10b) of the indoor unit (2a, 2b) that performs the defrosting operation to be in a closed state during the defrosting operation, and control the ventilation damper (10a, 10b) of the indoor unit (2a, 2b) that performs the heating operation to achieve a required amount of ventilation corresponding to a ventilation state of the room, and render the ventilation damper (10a, 10b) of the indoor unit (2a, 2b) that performs the defrosting operation to be in a closed state during the defrosting operation, and control the ventilation damper (10a, 10b) of the indoor unit (2a, 2b) that performs the heating operation to achieve a required amount of ventilation corresponding to a ventilation state of the room, and in a case where at least one of the refrigeration cycles is stopped, when starting a defrosting operation by another refrigeration cycle, start the defrosting operation after activating the stopped refrigeration cycle to start a heating operation, cause the stopped refrigeration cycle to continue the heating operation even after the refrigeration cycle on the defrosting side that has started the defrosting operation terminates the defrosting operation, and stop the refrigeration cycle on the defrosting side after terminating the defrosting operation. start the defrosting operation after activating the stopped refrigeration cycle to start a heating operation, cause the stopped refrigeration cycle to continue the heating operation even after the refrigeration cycle on the defrosting side that has started the defrosting operation terminates the defrosting operation, and stop the refrigeration cycle on the defrosting side after terminating the defrosting operation.