The investigation on the distribution of gas concentration during gas-purging in oil tank of a tanker is necessary for securing safety of life and preventation from gas explosion. The decisive factors governing gas diffusion are structural arrangement in tank, supplied volume and characteristics of gas and position of inlet and outlet. The relations between these factors and gas concentration are not yet sufficiently clarified, though several experiments were carried out for this purpose up to present. In this paper, variation of gas concentration in tank as the time proceeds is analyzed for one-dimensional model firstly, and dispersion coefficients are estimated by using the measured values of gas concentration. Next, two-dimensional analyses by finite element method are carried out to estimate the variations of the distributions of flow velocity and gas concentration in tank with time. By the results of analyses, it has been made clear that the interception for gas purging is caused by existence of circulated gas flow and dead zone in tank.