PURPOSE: To provide a control system which monitors the ambient pressure in such a closed environment as that in the cabin of an airplane and automatically changes the ambient pressure.

CONSTITUTION: Many optical fiber cables 30 are laid as signal transmitting means between the external and internal walls 22 and 24 of the body of an airplane, especially, around windows 12, doors, etc., having high possibility of being damaged by regarding the whole body of the airplane as one pressurized container. On the other hand, the relative change (line impedance change, etc.) of a signal from a first signal outputting means, such as the laser, etc., within a certain period of time is received and decided by means of a detecting and comparing means. Based on the decided signal change, a second signal (control signal) is generated. The second signal changes the ambient pressure in the enclosed environment and is received by means of a driving device which can respond to the second signal.