A simple theoretical analysis was carried out in order to clarify the general characteristics of pressure loss in exhaust gas flow induced when exhaust gas passes through a catalyst, which is generally used for the purification of exhaust gas emitted from automobile engines. Namely, Darcy's friction factor was applied to an equation, which is well known in the field of fluid dynamics, to determine the pressure drop of fluid flowing in a flow path. Using this friction factor, the air pressure difference between the upstream and downstream of the catalyst was calculated using the hydraulic diameter of the catalyst cell, the Reynolds number based on the hydraulic diameter of the catalyst cell, the length of the catalyst, and the density and velocity of the air flowing in the catalyst cell. Here, the flow of air is a laminar flow, which is assumed to be in the steady state, and the cross-sectional shapes of the catalyst cells examined are square, circle and equilateral triangle. Calculations were carried out for each shape, for which the mill number, cell number and open-area ratio of the catalyst were changed. In addition, the effect of air temperature on pressure loss was elucidated. On the basis of these calculations, the author clarified the pressure losses generated in catalysts having different cross-sectional shapes, as well as the difference between these pressure losses, under various conditions. The author supposes that the absolute values of these results are not highly accurate. However, in the early stage of designing new catalytic converter systems, this method is effective and can easily be used to estimate the magnitude of pressure loss in a short period of time, which is one of the fundamental characteristics of converter systems.