We studied the behavior of air-water interface in a horizontal tube caused by a step change of air flow above still water. As to the critical condition of wave generation, the experimental results are well explained by the theory of Kelvin-Helmholtz instability. This tendency remains true also in case of different surface tension. The lower limit of air velocity for slugging in a short time (1.75 sec) after the staring of the flow is about 3m/s greater than the results of Wallis which correspond to the transition from the steady wavy flow.