PURPOSE: To enable such accurate pressure proof expansion test as leaves no residual air in a pressurized head and a bending part of piping, etc., and as can drain dirts floating in a cylinder into the vacuum tank side without causing clogging.

CONSTITUTION: In this pressure proof expansion test device, the water in a balance water tank 3 is sent to a cylinder 1 side by a pressurizing pump 4 to pressurize and make it expand, and the water is returned to the balance water tank 3 by removing the pressure. A pressurized head 2 attached to the cylinder 1 is provided with an inlet side flow path 21 for injecting water and pressurizing the cylinder 1 and an outlet side flaw path 22 for draining the air and water out of the cylinder 1 side. The outlet side flaw path 22 is connected to a vacuum pump 62 by way of an air bleeder drain valve 6 and a vacuum tank 61. Before the cylinder 1 is filled with water by injection, the vacuum pump 62 bleeds the air out of the cylinder 1.