Provided is a pressure vessel able to sustain a circular shape without deforming to ellipsoidal shape due to machining of side port apertures therein, and maintain pressures as high as 800-1,200 PSI. The pressure vessel is intended for use in seawater desalination by reverse osmosis, and comprises a pipe body (1) formed by filament winding (FW), and is furnished with an occluding cover (2) at each end of the pipe body (1), the peripheral surface of the pipe body being furnished, inward from the occluding covers (2), with inflow/outflow parts (3) through which the fluid flows in or out. The pipe body (1) is constituted by a helical layer (4), hoop layers, and a seal layer (6) furnished as the innermost layer. The helical layer (4) is furnished with a divergent part (7) diverging away from the seal layer (6) and extending from a location outward from the occluding cover (2) to a location inward from the inflow/outflow part (3). The hoop layers are constituted by a first hoop layer (8) furnished between the divergent part (7) and the seal layer (6), and a second hoop layer (9) furnished at the outer circumference of the helical layer (4), and extending from a location outward from the occluding cover (2) to a location inward from the inflow/outflow part (3).