To provide a fluid merging part structure in pipe which is not affected by temperature difference, nor does cause thermal fatigue, even when fluid with temperature difference merges each other.

An inner pipe 6 is extended into a main pipe 4 from a branch pipe 5 arranged on the main pipe 4. A thermal sleeve 7 made of heat resistant material is inserted into the main pipe 4 along it at its position spaced from an inner wall of the main pipe 4. An opening end 6a penetrates a side wall of the thermal sleeve 7. The inner pipe 6 is communicated with the thermal sleeve 7.