A pressure vessel comprising: 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 to extend in a longitudinal direction; a body flange provided at at least one end side of the pressure vessel body in the longitudinal direction and having a circular cross-sectional shape; and a connecting member connecting the pressure vessel body and the body flange to each other, wherein the connecting member has a body-flange connected portion (24, 44) connected to the body flange and formed in a cylindrical shape, a pressure-vessel-body connected portion connected to the pressure vessel body, and a connecting portion connecting the body-flange connected portion (24, 44) and the pressure-vessel-body connected portion to each other and formed in a cylindrical shape with a shape changing to be gradually smaller from the pressure-vessel-body connected portion toward the body-flange connected portion (24, 44), wherein the body-flange connected portion (24, 44) has a circular cross-sectional shape, wherein the pressure-vessel-body connected portion is larger in outer shape than the body-flange connected portion (24, 44), and is formed in a cylindrical shape, characterised in that the connecting portion is formed in a conical, semi-spherical, or semi-ellipsoidal shape, the pressure-vessel-body connected portion has a circular cross-sectional shape, and the pressure-vessel-body connected portion is formed such that one end side of the pressure vessel body is configured to be inserted and fitted in the pressure-vessel -body connected portion. 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 to extend in a longitudinal direction; a body flange provided at at least one end side of the pressure vessel body in the longitudinal direction and having a circular cross-sectional shape; and a connecting member connecting the pressure vessel body and the body flange to each other, wherein the connecting member has a body-flange connected portion (24, 44) connected to the body flange and formed in a cylindrical shape, a pressure-vessel-body connected portion connected to the pressure vessel body, and a connecting portion connecting the body-flange connected portion (24, 44) and the pressure-vessel-body connected portion to each other and formed in a cylindrical shape with a shape changing to be gradually smaller from the pressure-vessel-body connected portion toward the body-flange connected portion (24, 44), wherein the body-flange connected portion (24, 44) has a circular cross-sectional shape, wherein the pressure-vessel-body connected portion is larger in outer shape than the body-flange connected portion (24, 44), and is formed in a cylindrical shape, characterised in that the connecting portion is formed in a conical, semi-spherical, or semi-ellipsoidal shape, the pressure-vessel-body connected portion has a circular cross-sectional shape, and the pressure-vessel-body connected portion is formed such that one end side of the pressure vessel body is configured to be inserted and fitted in the pressure-vessel -body connected portion. a body-flange connected portion (24, 44) connected to the body flange and formed in a cylindrical shape, a pressure-vessel-body connected portion connected to the pressure vessel body, and a connecting portion connecting the body-flange connected portion (24, 44) and the pressure-vessel-body connected portion to each other and formed in a cylindrical shape with a shape changing to be gradually smaller from the pressure-vessel-body connected portion toward the body-flange connected portion (24, 44), wherein the body-flange connected portion (24, 44) has a circular cross-sectional shape, wherein the pressure-vessel-body connected portion is larger in outer shape than the body-flange connected portion (24, 44), and is formed in a cylindrical shape, characterised in that the connecting portion is formed in a conical, semi-spherical, or semi-ellipsoidal shape, the pressure-vessel-body connected portion has a circular cross-sectional shape, and the pressure-vessel-body connected portion is formed such that one end side of the pressure vessel body is configured to be inserted and fitted in the pressure-vessel -body connected portion.