PURPOSE: To stably maintain the performance of a dead plate, and reduce an air leakage while performing an appropriate maintenance in response to the condition by monitoring the operating condition of the sealing surface of a dead plate by measuring a vertical movement of the dead plate.

CONSTITUTION: A vertical motion of a dead plate 1 is always detected by engaging a connecting rod 4 for position detection with the dead plate 1, and the operating condition of the sealing surface of the dead plate 1 is monitored. For the detection of the vertical movement, a mechanical displacement of the vertical movement is converted into an electric signal by a transmitter 5 for vertical motion detection, and continuously input in a computer, and when the input signal value exceeds a specified range, it is notified by an alarm or display. By this method, the operating condition of the dead plate 1 can always be monitored, and an appropriate maintenance can be performed in response to the condition. At the same time, an air leakage can be reduced by stably maintaining the performance of the dead plate 1.