Abstract: The ground surrounding a liquified gas storing underground tank is excavated into a form of a columnar cavity or wall shaped cavity, and then a synthetic resin bag or envelope is placed therein, after which granular materials such as sand or gravel is filled therein, thus forming a water permeable body of a columnar shape or wall-like shape. Then, water is injected into the water permeable body from its bottom and then discharged from the top of the body, thereby warming the peripheral ground of the underground tank for preventing freezing thereof. An apparatus for preventing the freezing of a liquified gas storing underground tank is also disclosed, which includes water feeding and discharging devices provided for the aforesaid water permeable body.