To confirm an underwater welding state easily with eyes by applying a hydraulic pressure equal to the water depth to one side of an underwater object and applying the atmospheric pressure to a predetermined region including a welding line on the other side.

In the case of a pipe, water is injected into the pipe and a part thereof is opened thus applying a hydraulic pressure equal to the water depth. On the other hand, the atmospheric pressure is applied to a predetermined region including a welding line on the other side. An inspection container 1 is thereby fixed while closing all valves and then pressurized gas is supplied from an atmospheric pressure valve 5 by opening a lower drainage valve 4. Consequently, water in the container 1 is drained through the drainage valve 4 and the gas is stored in the upper part of the container 1. Subsequently, an atmospheric pressure valve 6 is opened to supply the atmospheric pressure into the container 1 through a pressure resistant tube 7. Consequently, hydraulic pressure acts on the inside of a welding line and the atmospheric pressure prevails on the other side. Since water leaks through an incompletely welded part due to the pressure difference, incomplete welding can be checked.