To upgrade an intake/drainage port of a pool with a water circulation system and a drainage system.

An intake/drainage port 2 formed on the bottom or the side surface of a pool and an intake/drainage pipe 3 connected thereto are provided while an intake/drainage port cover 4 is mounted on port 2. An on/off valve of the intake/drainage pipe 3 and the intake/drainage port cover 4 are interlocked. In the case when the intake/drainage port cover 4 is removed from the port 2, the intake/drainage pipe 3 is closed with the on/off valve. In the case when the port cover 4 is fitted in the port 2, the on/off valve is opened, thereby forming an interlocking mechanism as described above. In the case when the cover 4 is removed from a specified position, this on/off interlocking mechanism operates, thereby closing the intake/drainage pipe and exerting a great role which prevents the generation of an unexpected accident.