Measurements of time-varying spatial liquid holdup distributions and interfacial profiles were carried out for the transition flow region between plug and annular flows using the liquid with three types of liquid viscosity νι=1, 10 and 20 cSt. Time-spatial maps of interfacial behavior were drawn and close inspection revealed that two types of waves appeared for each liquid viscosity, namely one is disturbance wave with small height, width and velocity. The other is huge wave with large height, width and velocity. Furthermore, it was clarified that no wave continuing in the measuring section appears in higher gas velocity and lower liquid velocity conditions for νι=10 and 20 cSt and the interface was covered with ripple waves. To clarify the differences between huge wave and disturbance wave definitely, a wave vein analysis was carried out. Probability distributions of these flow parameters were determined. The characteristics in these probability distributions revealed that two types of distributions existed for both of wave width and velocity and these distributions corresponded to those of huge wave and disturbance wave, respectively.