PURPOSE: To simplify pressurizing in a pressure resistance test after the modification of a large diameter pipe by increasing the number of jackets.

CONSTITUTION: Since a piece of ice plug is not strong enough in a pressure resistance test accompanied by the modification of a large diameter pipe, back pressure is applied to what is opposite to the pressurizing side to make the plug resistant against the pressure of the pressurizing side. In other words, a needle valve on the outlet side of a water pressure pump for applying the back pressure to let escape the water equivalent to a pressure rise. Subsequently, it is confirmed that the ice plug has grown by more than the preselected breadth. Then, the predetermined back pressure is applied across jackets A and B to apply the predetermined pressure from the pressurizing side. The ice plug is thus prevented from sliding with a piece of jacket even at the pressure higher than what would cause the ice plug to start sliding. Accordingly, this makes it possible to simplify pressurization in a pressure resistance test after the modification of a large diameter pipe.