An air conditioning system comprising: an air conditioner including an outdoor unit and indoor units belonging to a common control system, the indoor units being installed in a common room; motion detectors each provided to one of the indoor units, and configured to detect presence of a person in the room; and a controller (37, 45) configured to cause the air conditioner to suspend an air conditioning operation if the room is vacant, characterized in that while the air conditioning operation is suspended, the controller (37, 45): obtains a detection time period sum (T2a-c) by summing, for each of the motion detectors, time periods in which the presence of the person is detected, if at least one of the motion detectors, each provided to one of the indoor units, starts to detect the presence of the person during the suspension of the air conditioning operation; and causes the air conditioner to resume the air conditioning operation if any one of detection time period sums (T2a-c), including the detection time period sum and each for one of the motion detectors, reaches a reference value within a predetermined time period after the at least one motion detector starts to detect the presence of the person, the detection time period sums (T2a-c) includes: a case when any one of the motion detectors continuously indicates the result of detection "in use" for a time period of the reference value; and a case in which, even when any one of the motion detectors indicates the result of detection "in use" over interrupted time periods, the sum of the interrupted time periods, indicating the result of detection "in use", has reached the reference value before the elapsed time period being measured reaches the predetermined time period. an air conditioner including an outdoor unit and indoor units belonging to a common control system, the indoor units being installed in a common room; motion detectors each provided to one of the indoor units, and configured to detect presence of a person in the room; and a controller (37, 45) configured to cause the air conditioner to suspend an air conditioning operation if the room is vacant, characterized in that while the air conditioning operation is suspended, the controller (37, 45): obtains a detection time period sum (T2a-c) by summing, for each of the motion detectors, time periods in which the presence of the person is detected, if at least one of the motion detectors, each provided to one of the indoor units, starts to detect the presence of the person during the suspension of the air conditioning operation; and causes the air conditioner to resume the air conditioning operation if any one of detection time period sums (T2a-c), including the detection time period sum and each for one of the motion detectors, reaches a reference value within a predetermined time period after the at least one motion detector starts to detect the presence of the person, the detection time period sums (T2a-c) includes: a case when any one of the motion detectors continuously indicates the result of detection "in use" for a time period of the reference value; and a case in which, even when any one of the motion detectors indicates the result of detection "in use" over interrupted time periods, the sum of the interrupted time periods, indicating the result of detection "in use", has reached the reference value before the elapsed time period being measured reaches the predetermined time period. obtains a detection time period sum (T2a-c) by summing, for each of the motion detectors, time periods in which the presence of the person is detected, if at least one of the motion detectors, each provided to one of the indoor units, starts to detect the presence of the person during the suspension of the air conditioning operation; and causes the air conditioner to resume the air conditioning operation if any one of detection time period sums (T2a-c), including the detection time period sum and each for one of the motion detectors, reaches a reference value within a predetermined time period after the at least one motion detector starts to detect the presence of the person, the detection time period sums (T2a-c) includes: a case when any one of the motion detectors continuously indicates the result of detection "in use" for a time period of the reference value; and a case in which, even when any one of the motion detectors indicates the result of detection "in use" over interrupted time periods, the sum of the interrupted time periods, indicating the result of detection "in use", has reached the reference value before the elapsed time period being measured reaches the predetermined time period. a case when any one of the motion detectors continuously indicates the result of detection "in use" for a time period of the reference value; and a case in which, even when any one of the motion detectors indicates the result of detection "in use" over interrupted time periods, the sum of the interrupted time periods, indicating the result of detection "in use", has reached the reference value before the elapsed time period being measured reaches the predetermined time period.