The purpose of this study is to propose a modified average flow model that can be used for mechanical seals with parallel sealing faces, and clarify their lubrication characteristics. The general average flow model established by Patir et al. could not consider the effects of surface roughness between parallel faces. In the present model, the flow factors with considering side leakage effects between parallel lubricating faces are implemented. In addition, the model allows one to consider the additional expected hydrodynamic pressure generated owing to the relative motion of rough surfaces. The flow factors are evaluated as the average for several different surfaces with the same statistical properties. In this first report, the effects of the root mean square roughness on the lubrication characteristics are investigated, assuming that the probability density function of the roughness follows the normal distribution. The results show that as the root mean square roughness increases, the flow rate increases. In addition, the optimal roughness for the friction coefficient is changed with the rotational speed.