To provide a method for forming an underground impermeable layer capable of easily and positively forming the impermeable layer capable of cutting off an underground space at a temperature of 100°C or more from an interior of a rock mass excluding the space.

A reactant containing at least CO2 and H2O is supplied into an underground permeable layer 22 at a temperature of 100°C or more, the reactant is reacted with the rock mass positioned in the underground permeable layer 22 to form carbonate and clay minerals, and by the carbonate and clay minerals, the impermeable layer 23 cutting off at least one part from other parts is formed in the underground permeable layer 22.