In this report, continued from the 1st report, the authors treated experimentally the flow of air-water mixture in the horizontal pipes which have a round branch pipe jutting into the main of rectangular section. First, an experiment on the flow of air-water mixture in a straight rectangular pipe was conducted, and its flow pattern, frictional loss, volume fraction of water and relative velocity were studied and the results were compared with those of a round pipe. The experiment has shown the coefficient of head loss ξb' and ξc' at a cross-branch section respectively as follows. [numerical formula] where ξb' and ξc'=coefficients of head loss at a cross branch section for branch pipe and main pipe respectively. wma' and γma'=real mean velocity and real mean specific weight at a cross branch section which is contracted by the jut of branch pipe end, respectively. wm=mean velocity. γm=mean specific weight. B and D'=height of a side and equivalent diameter of main rectangular pipe respectively. subscripts a, b and c=sections of upper main, branch and lower main pipes respectively.