Evaluation of the mean diameter of bubbles released from a hole is indispensable in designing gas distributors and for numerical simulation of bubbly flows in a Fischer-Tropsch reactor in a GTL plant, which is now being developed by JOGMEC. The objective of the present study is to develop a correlation for evaluating the mean diameter of bubbles released into a column through a hole. A simple correlation for the bubble diameter is deduced by modifying the Davidson & Schuler model based on the fact that a coalescence of two or three bubbles takes place just after the detachment. The proposed correlation is applicable to the evaluation of the mean diameter of bubbles discharged from two holes when the ratio of a pitch between holes to a bubble diameter is larger than 1.7.