An image processing method for measuring the instantaneous two-dimensional (2D) local void fraction at high bubble number density was proposed. It was based on computing a correlation between the locally averaged brightness of the bubble plume images and the real void fraction. The method was tested using the 2D distribution of local void fraction from visualized image, where a correlation between the locally averaged brightness and the projection void fraction was derived by introducing an ideal of overlapping probability of the bubble-images under various conditions. This research was used to study the behavior of the bubbly flows with aspect ratio of 1.0 1.5, and 2.0. The present method was applied to the instantaneous measurement. The void fraction for bubble plumes of three different aspect ratios with three different bubble generating flow rates were measured for the clarification of the effect of the bubble on the behavior of the bubbly flows. Dependency of the void fraction on the gas flow rate was measured and compared to other results from the literature. In the range of our experiments, it was observed that not only does an aspect ratio of the tank affect the structure of the plume, but also the gas flow rate in the tank affects the structure of the plume.