A pressure vessel comprising a pressure vessel body having a rectangular cross-sectional shape and formed to extend in a direction of flow of fluids; a first lid member which is provided on one longitudinal end surface of the pressure vessel body; and a second lid member which is provided on the other longitudinal end surface of the pressure vessel body, does not have an opening and closing structure, and is unopenable, wherein the pressure vessel body includes a first flow channel which is formed in a longitudinal direction of the pressure vessel body and through which a first fluid is caused to flow, a second flow channel which is formed in the longitudinal direction of the pressure vessel body and through which a second fluid is caused to flow, a first-fluid inlet-outlet port which is provided in the one longitudinal end surface of the pressure vessel body and connects with the first flow channel and through which the first fluid is caused to flow in or out, a second-fluid inlet-outlet port which is provided in the other longitudinal end surface of the pressure vessel body and connects with the second flow channel and through which the second fluid is caused to flow in or out, an opening portion which is provided in the one longitudinal end surface of the pressure vessel body and connects with the second flow channel, characterised in that the pressure vessel further comprises a closing member which closes the opening portion in a demountable manner, wherein the first lid member has an opening and closing structure, and is openable and closable, and a replacable in-channel component which is provided in the second flow channel, and the second flow channel is formed such that the in-channel component is removable and insertable from the the opening portion. a pressure vessel body having a rectangular cross-sectional shape and formed to extend in a direction of flow of fluids; a first lid member which is provided on one longitudinal end surface of the pressure vessel body; and a second lid member which is provided on the other longitudinal end surface of the pressure vessel body, does not have an opening and closing structure, and is unopenable, wherein the pressure vessel body includes a first flow channel which is formed in a longitudinal direction of the pressure vessel body and through which a first fluid is caused to flow, a second flow channel which is formed in the longitudinal direction of the pressure vessel body and through which a second fluid is caused to flow, a first-fluid inlet-outlet port which is provided in the one longitudinal end surface of the pressure vessel body and connects with the first flow channel and through which the first fluid is caused to flow in or out, a second-fluid inlet-outlet port which is provided in the other longitudinal end surface of the pressure vessel body and connects with the second flow channel and through which the second fluid is caused to flow in or out, an opening portion which is provided in the one longitudinal end surface of the pressure vessel body and connects with the second flow channel, characterised in that the pressure vessel further comprises a closing member which closes the opening portion in a demountable manner, wherein the first lid member has an opening and closing structure, and is openable and closable, and a replacable in-channel component which is provided in the second flow channel, and the second flow channel is formed such that the in-channel component is removable and insertable from the the opening portion. a first flow channel which is formed in a longitudinal direction of the pressure vessel body and through which a first fluid is caused to flow, a second flow channel which is formed in the longitudinal direction of the pressure vessel body and through which a second fluid is caused to flow, a first-fluid inlet-outlet port which is provided in the one longitudinal end surface of the pressure vessel body and connects with the first flow channel and through which the first fluid is caused to flow in or out, a second-fluid inlet-outlet port which is provided in the other longitudinal end surface of the pressure vessel body and connects with the second flow channel and through which the second fluid is caused to flow in or out, an opening portion which is provided in the one longitudinal end surface of the pressure vessel body and connects with the second flow channel, characterised in that the pressure vessel further comprises a closing member which closes the opening portion in a demountable manner, wherein the first lid member has an opening and closing structure, and is openable and closable, and a replacable in-channel component which is provided in the second flow channel, and the second flow channel is formed such that the in-channel component is removable and insertable from the the opening portion.