A new binder segregation model incorporating solution diffusion was proposed for convective drying of porous solid without shrinkage by expansion of the authors' previous model without diffusion. Results calculated with the new model revealed that the assumption of negligible diffusion in the previous model was quantitatively appropriate for the previous experimental results, and that the diffusion caused a relaxation of binder segregation. The binder segregation curve calculated with the new model was only valid when the drying rate curve was known. When the drying rate curve was unknown, the range in which the influence of diffusion was negligible could be obtained by use of the "solution diffusion parameter" introduced in this study.