Due to the large dissipation of magnetostatic waves, it appears that there are difficulties in the application of magnetostatic wave devices to microwave systems. In this paper, a theoretical estimation makes it clear that previously described large losses are based mainly on the mismatch of impedance between microwave systems and the ferrimagnetic devices. In this analysis, the authors discuss the characteristic impedance of a layered structure, which consists of a ferrimagnetic slab and a dielectric slab having a large permittivity. The optimum parameter for precise impedance matching of the layered structure is determined.