To provide a method can prevent the collapse of porous walls when tests are executed under condition of fresh water in the hole of a borehole excavated in the ground through the use of a slurry and provide a casing used for the same and a testing method executed in the hole in which the method is executed.

The method for preventing the collapse of porous walls when tests are executed under condition of fresh water in the hole of a borehole 20 excavated in the ground through the use of a slurry is provided with a casing installation process S10 for inserting the casing 10, which is provided with a steel pipe 12 and a freely expanding and contracting packer 14 provided for the external circumference of the steel pipe 12, into a prescribed depth in the hole of the borehole 20; a packer expansion process S20 for expanding the packer 14, which is provided for the casing 10 inserted in the hole of the borehole 20, in such a way as to come into tight contact with the hole wall of the borehole 20: and a substituting process S30 for substituting slurries below and inside the steel pipe 12 with fresh water by an expanded packer 14.