Mixed stream pipeline structure

High temperature water (low temperature water) in a branched pipe and low temperature water (high temperature water) in a main pipe are jointed at a junction, to form an intermediate temperature fluid in the main pipe. The branched pipe having a restricted portion is connected, by welding, to the main pipe at the junction. Static pressure is reduced at the restricted portion due to the restriction by so much as the increase of the flow speed. A thermal sleeve is protruded at the exit of the branched pipe in the same axial direction with the branched flow. A bypass structure is disposed for communicating from the main pipe to the branched pipe after joining of flows. The intermediate temperature fluid partially flows in the bypass pipe by pressure reduction at the restricted portion. The intermediate temperature fluid in the bypass pipe is mixed with the fluid in the branched pipe with no great temperature difference, and the temperature difference of the fluid in the branched pipe relative to that in the main pipe is reduced. This suppresses temperature fluctuation on the wall surface at the junction by decreasing the temperature difference at the joint flow mixing portion of the high temperature water pipeline and the low temperature water pipeline, thereby enabling to suppress thermal fatigue of the material. (I.N.)