A pressure vessel includes a pressure vessel body provided with a flow channel through which a fluid is caused to flow, having a rectangular cross-sectional shape, and formed in an elongated shape, and a circular body flange provided on at least one longitudinal end side of the pressure vessel body, the pressure vessel body has a fluid inlet-outlet port which is provided on the one longitudinal end side and the body flange side of the pressure vessel body and connects with the flow channel and through which the fluid is caused to flow in or out, and the pressure vessel further includes an inlet-outlet header which is formed between the pressure vessel body and the body flange as a closed space connecting with the fluid inlet-outlet port and which the fluid is caused to flow into and out of.