The dimension of throat is one of the most important problems concerning the design of tank furnace, because it is on the closest relationship to the acquisition of homogeneity of the glass and the long life of the throat refractory. Nevertheless, as none of the theoretical rules in respect to this problem has been found hitherto, the dimension of throat is determined only by experiences. But the optimum condition of throat with the simple type may be found by theoretical considerations alone.In consequence of the mathematical treatments, the velocity distribution formula of the throat current was derived, and then it was found that the velocity of throat current varies remarkably by the height of the throat and obtains a minimum value at a suitable height and width. Assuming that this dimension is the optimum condition, the following formula which determines the optimum dimension of throat was given.f1/4hopt=2.88(Wηl/ρ0ρbgkΔθ)1/4Moreover, as a result of calculation of the throat current under the optimum condition, it was concluded that the velocity of the current under this condition is almost determined only by the height of throat, but not nearly affected by the corresponding width and the speed of pull. Results of these calculations are in fairly good agreement with the observed and calculated values which were reported already by the other investigators. However, it was concluded that the theoretical value of the optimum height is obviously lower than the practical dimension.