For estimation of the gas emission amount from a storage tank for storing a liquid containing the volatile compounds therein, preconditions involving a tank shape of the fixed roof type storage tank and a working pressure of a relief valve are set in a first step, and initial conditions for storage volumes, temperatures, and pressures of the liquid and a gas inside the storage tank are set in a second step. In a third step, an inflow or outflow heat quantity to or from the liquid and the gas inside the storage tank is determined so that a mass transfer amount between the liquid and the gas is obtained through CFD analysis. In a fourth step, whether or not the relief valve is actuated and the gas emission amount through the relief valve are obtained based on the mass transfer amount.