The friction factor (Pressure drop coefficient) λ of the metallic wire was obtained by measuring the drop in pressure at different values of velocities of the air in a given length of a tube with very thin rectangular cross section, in which the surface of the bottom plane was covered with the metallic wires or with the flat model wires set in parallel and evenly.The relationship between λ and Re (Reynolds Number) plotted directly on the logarithmic scale, was linear below Re_??_ 1000 and started to deviate upward from this point.At this point, or the critical Reynolds Number Rc, the flow of the air begins to change from laminar to turbulent and the mean velocity of the air corresponds to about 11m/s in actual size of the metallic wire in use. These results are agreed with those shown by the visual observation in the transparent tube in our previous report1).Comparison between the frictional factor λt obtained from the actual metallic wire and the friction factor λp obtained from the flat model wire as shown in Fig. 5, showed that λt, was 16% larger than λp.