Although an intergranular superconducting system is definitely different from a conventional type-II superconductor in several aspects such as flux shape, flux size, and flux behaviors, its magnetic properties have been described by the critical state model in an equivalent manner to those of a type-II superconductor's. Instead of the critical state model, we suggest another primitive model which is able to describe the magnetic properties of an intergranular system. We have made a model calculating the magnetic susceptibility of a superconducting coaxial cylindrical multi-shell structure zero-field-cooled under an external field and have applied the model to susceptibility versus temperature data of a YBa2Cu3Ox polycrystal. Calculation results by this model concurred well with the experimental data. A physical view point of this model is that an intergranular superconducting system zero-field-cooled under an external field can be treated to be a hypothetical coaxial multi-shell structure.