The collection characteristics of a solar air heater with a built-in net, in which a metallic net was inserted parallel in the space between the transparent cover plate and the absorber, were theoretically analyzed, The working fluid (air), which entered the solar air heater at a uniform inlet velocity and a constant inlet temperature, was divided into two layers (upper and lower layer) by the metallic net, Solar radiation impinges uniformly from above, passing through the transparent cover plate and being absorbed partly by the net and partly by the absorber. For the present solar air heater, the continuity, momentum and energy equations were derived under several assumptions and numerically solved. The calculations mere performed for the cases of eight kinds of metallic nets which had different transmissivities, under different combinations of mass now rate of air and incident solar radiation, Also, the cases of a conventional solar air heater and a solar air heater without a net were theoretically analyzed, The calculated results were compared with the experimental data reported in a previous paper, and the effects of the net transmissivity, net setting position and solar air heater length on temperature distributions within the air layer and the collection efficiency were examined. The calculated results show fairly well the tendencies of the experimental data, and the several assumptions made and the calculation method used in the theoretical analysis are found to be valid.