The ventilation pipe 60 having the openings at both the ceiling side and the bottom side is provided in the area where the rectification boards 50a to 50f are arranged, thereby nitrogen released from water (nitrogen released from water based on that gas dissolved in water is substituted by oxygen) can be moved to the ceiling side of the retainer body through the ventilation pipe 60. Thereby, nitrogen can be effectively exhausted from the exhaust opening 30a, thus it can be restrained that nitrogen is accumulated in the retainer body 10. Accordingly, it can be restrained that oxygen concentration (partial pressure) in the retainer body 10 decreases. Since oxygen quantity dissolved in water is increased, gas dissolved in water can be effectively substituted.