A pressure vessel (10, 40) 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 (22, 42) connecting the pressure vessel body and the body flange to each other, wherein the connecting member (22, 42) has a body-flange connected portion (24, 44) connected to the body flange, having a circular cross-sectional shape, and formed in a cylindrical shape, a pressure-vessel-body connected portion connected and joined to the pressure vessel body and formed in a cylindrical shape, and a connecting portion connecting the body-flange connected portion (24, 44) and the pressure-vessel-body connected portion to each other and formed with a shape changing to be gradually smaller from the pressure-vessel-body connected portion toward the body-flange connected portion (24, 44), the pressure-vessel-body connected portion has a rectangular cross-sectional shape, the connecting portion is formed such that one end thereof connected to the pressure-vessel-body connected portion is formed in a rectangular shape while another end thereof connected to the body-flange connected portion (24, 44) is formed in a circular shape and a cross-sectional shape of the connecting portion gradually changes from the rectangular shape into the circular shape from the one end, connected to the pressure-vessel-body connected portion, toward the other end, connected to the body-flange connected portion (24, 44), characterised in that the pressure-vessel-body connected portion is larger in outer shape than the pressure vessel body and the body-flange connected portion (24, 44; 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; and 11 in a side view of the pressure vessel (10, 40), the pressure vessel body is smaller in outer shape than the body-flange connected portion (24, 44). 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 (22, 42) connecting the pressure vessel body and the body flange to each other, wherein the connecting member (22, 42) has a body-flange connected portion (24, 44) connected to the body flange, having a circular cross-sectional shape, and formed in a cylindrical shape, a pressure-vessel-body connected portion connected and joined to the pressure vessel body and formed in a cylindrical shape, and a connecting portion connecting the body-flange connected portion (24, 44) and the pressure-vessel-body connected portion to each other and formed with a shape changing to be gradually smaller from the pressure-vessel-body connected portion toward the body-flange connected portion (24, 44), a body-flange connected portion (24, 44) connected to the body flange, having a circular cross-sectional shape, and formed in a cylindrical shape, a pressure-vessel-body connected portion connected and joined to the pressure vessel body and formed in a cylindrical shape, and a connecting portion connecting the body-flange connected portion (24, 44) and the pressure-vessel-body connected portion to each other and formed with a shape changing to be gradually smaller from the pressure-vessel-body connected portion toward the body-flange connected portion (24, 44), the pressure-vessel-body connected portion has a rectangular cross-sectional shape, the connecting portion is formed such that one end thereof connected to the pressure-vessel-body connected portion is formed in a rectangular shape while another end thereof connected to the body-flange connected portion (24, 44) is formed in a circular shape and a cross-sectional shape of the connecting portion gradually changes from the rectangular shape into the circular shape from the one end, connected to the pressure-vessel-body connected portion, toward the other end, connected to the body-flange connected portion (24, 44), characterised in that the pressure-vessel-body connected portion is larger in outer shape than the pressure vessel body and the body-flange connected portion (24, 44; 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; and 11 in a side view of the pressure vessel (10, 40), the pressure vessel body is smaller in outer shape than the body-flange connected portion (24, 44).