To efficiently and completely replace water in a tank by fresh water with simple construction.

A tank 12 is connected to a water service pipe 10 through an inlet pipe 14 and an outlet pipe 16 and, at the same time, in the case an earthquake occurs, earthquake sensing automatic opening and closing valves 18 and 20 for interrupting the pipes 14 and 16 are provided to the pipes 14 and 16 to constitute an emergency water storage device. The inside of the tank 12 is formed so as to have a circular section and the same inside diameter in the longitudinal direction. An inlet pipe 35 and an outlet pipe 39 connected respectively to the pipes 14 and 16 and extended to the direction of diameter inside the tank are provided to both longitudinal ends of the tank 12. Two inlet ports opened in the opposite direction at right angles to the center axis of the tank 12 are provided respectively to the basic end and front end of the inlet pipe 35, and two outlet ports opened in the opposite direction on the center axis are provided to the outlet pipe 35.