To inform an operator of the fact that a sash is opened at an extremely high speed by alarm by calculating the moving speed of a sash door and generating the alarm signal when the calculated speed exceeds preset speed.
A room controller receives a signal indicating the volume of air exhausted by a gas hood controller 20 and the same signal as that from an exhaust air flow sensor A1 indicating the volume of air exhausted apart from the gas hood exhausted air through a main exhausting system. The room controller calculates the difference between the total flow rate entering to the room and the total flow rate exhausted from the room by the signals. The differential pressure in the room is controlled to slightly lower than that of a reference space outside of the room, to 0.01-0.1inch in hydraulic pressure. When the opening speed of the sash reaches ≥1.5feet/sec, audible signal, visible signal or both signals are generated. Based on the intermittent alarm sound generated by the alarm 44, the moving speed of opening sash door is made slow by the operator.