Internal pressure vessel

Provided is an internal pressure vessel which is not deformed into an elliptical shape by hole drilling at a side port but maintains a precise circular shape so as to maintain a high pressure of between 800 PSI and 1,200 PSI. In the internal pressure vessel, which has a pipe body (1) formed by an FW method to be used in a seawater desalination process using a reverse osmosis membrane method or the like, a closing lid (2) is disposed at both ends of the pipe body (1). An inlet and outlet portion (3) is disposed to introduce and withdraw a fluid at an inner side of the closing lid (2) at a circumferential surface of the pipe body (1). The pipe body (1) is composed of a helical layer (4) and a hoop layer, and a sealing layer (6) which is disposed at an innermost layer. A separated portion (7) is disposed at the helical layer (4) away from the sealing layer (6) and across from an outer side position of the closing lid (2) toward an inner side position of the inlet and outlet portion (3). The hoop layer is configured from a first hoop layer (8) which is disposed between the separated portion (7) and the sealing layer (6), and a second hoop layer (9) which is disposed at an outer circumference of the helical layer (4) across from the side position of the closing lid (2) toward the inner side position of the inlet and outlet portion (3).