PURPOSE: To reduce both initial cost and running cost by a method wherein constant air flow operation and variable air flow operation are alternately performed in response to the load conditions to zones with large load fluctuation such as cuisine, dining hall and the like.

CONSTITUTION: Constant air flow operation and variable air flow operation are alternately performed in response to the load conditions to zones with large load fluctuation such as cuisine, dining hall and the like in an air-conditioning system, which controls the supply air flow rate and exhaust air flow rate are controlled by a computer. Concretely, various sensors to detect the load of said zones and signal converters are made into an integral body and the various sensors are connected to a single transmission line. The signals sent from the various sensors are processed at a transmission control unit and a general-purpose micro-computer so as to send control signals to a controller to control the supply air flow rate and exhaust air flow rate in order to alternately perform constant air flow operation and variable air flow operation at the air-conditioning of a zone with sharp load fluctuation under computer-control. Due to the above- mentioned mode of operation, the duration of constant air flow operation is shortened, resulting in reducing the running cost under small load.