To immediately cope with a dangerous event such as suction of a foreign matter like a human body at a drain outlet at the bottom of a pool, to thereby prevent an accident.

In a pool 1 in which water drained from a drain outlet 3 of the pool 1 by means of a circulating pump 5, is circulated and reused, a sensor 11 is arranged on a rear surface of a water-permeable cover 1 covering the drain outlet 3 or in the vicinity thereof. The sensor 11 is formed of a load cell, and activated by partial or entire rearward displacement of the cover 7. Further, a control circuit is provided, which generates a signal for activating a danger preventing means when it is determined that an abnormal event of covering of the drain outlet 3 with a foreign matter has occurred, based on an input signal indicative of a change in pressure detected by the sensor 11.