To achieve uniform thickness of coated films using non-Newtonian fluids, we are often required well-designed geometries of flow channel in a slot die. This is the case especially for fluids with the properties of both shear thinning and yield stress which is non-negligible in a range of low shear rate such as in an intermittent die coating. In the present work, flow in the slot die has been studied using Herschel-Bulkley fluid expressing both shear thinning and yield stress. The Herschel-Bulkley fluid includes power-law and Bingham fluid models comprehensively. This fluid model is applicable to many real fluids. We derive simplified equations for the outflow distribution as functions of power-law index, yield stress and the geometry parameters of the die. Furthermore, we propose a useful designing method for predicting the optimum geometry so as to guarantee uniformity of outflow from the die slot, based on the Herschel-Bulkley fluid. The value of this method is confirmed by comparing it with experimental results and with previous theoretical results for the power-law and Bingham fluid models.