PURPOSE: To easily prepare a center pipe having a full-flow passage and a bypass flowline and to stably secure the flowline area of the bypass flowline, by forming the bypass flowline by in-furnace soldering.

CONSTITUTION: A recessed groove 30' having a rectangular crossectional area for a bypass flowline 30 is formed to the peripheral surface of a metal pipe 22A having a relatively thick wall having a piercing bore 29' for a full-flow passage 29 provided to the center part thereof along the axial direction and step parts 41, 41 are formed to both side edges of the opening of said receissed groove 30'. On the other hand, an almost flat metal plate 22B having a width coinciding with the interval of said step parts 41, 41 is formed of a material similar to that of the above mentioned pipe 22A and one surface thereof is formed into a circular arc surface continuing to the peripheral surface of the above mentioned pipe 22A. This plate 22B is inlaid with and arranged to the above mentioned step parts 41, 41 through a solder material 42 forming a frame shape having two pieces bent at both sides thereof and, thereafter, the solder material is heated to a predetermined temp. to integrally connect the above mentioned pipe 22A and the plate 22B.