The safety design of nuclear plants against an earthquake is an important issue in the safety of nuclear reactors. Many studies for the earthquake-resistance of the nuclear plants has been performed for a structural strength of the nuclear power plants. On the other hand, although the gas-liquid two-phase flow in the nuclear power plants has important effects on the behavior of nuclear power plants, including the power of the reactor core, there is little knowledge of the behavior of the two-phase flow under the earthquake. For example, the bubbly flow behavior under a flow rate fluctuation caused by the earthquake acceleration is unclear. It is necessary to clarify the two-phase flow behavior under the earthquake conditions. To develop the prediction technology of two-phase flow dynamics, the detailed two-phase flow simulation code with an advanced interface tracking method (TPFIT) was expanded to the two-phase flow simulation under earthquake accelerating conditions. The purpose of the study is to clarify the behavior of the gas-liquid two-phase flow under the earthquake conditions and to use the results in order to compare them with the numerical simulation results. Especially, the bubble behavior in the two-phase flow, diameter, shape, and velocity of bubbles which are affected by the oscillation of the earthquake is investigated. In the experiment, the flow was bubbly flow and/or plug flow in a horizontal pipe. The working fluids were water and nitrogen gas. The water was driven by a pump and the flow rate fluctuation was given by an oscillator attached to the main flow loop. The frequency of the flow rate fluctuation was taken between 0.5Hz and 10Hz. The behavior of the horizontal gas-liquid two-phase flow under the flow rate fluctuation was measured by image processing using a high-speed video camera and PIV at the test section. The pressure sensors were installed at the inlet of the mixer and the outlet of the test section. As a result, the bubble/gas plug behavior mechanism under the flow rate fluctuation was obtained. The deformation of the gas-liquid interface was caused by the time change of the relative velocity between the gas and liquid phases.