In the main fuel pipe (main nozzle) of the carburetor of gasoline engine, a little air is let into the fuel stream, in general. This two-phase flow streams into the venturi-tube. The author's experiments are made with simple models to elucidate the basic phenomena of the confluence of air and fuel in a small tube, that is, the flow pattern of a two-phase flow, the pressure drop caused by the confluence, the two-phase flow friction, and the relaxation oscillation. In 1st Report, experimental apparatus and flow pattern are described. Vertical tubes are 6, 3, 2 mm of diameter. Liquid is tap water in the most part of experiments, and in some part of them, is distilled water, gasoline, solvent, or surface active agents solution, Unless the air holes are large, stable fish-scale type slug flow appears in some range of air and liquid. This is attributed not to the effect of so-called entrance length but to that of the small diameter of tube.