Based on the concept of effective flux pinning force proposed by the present authors, a method desoribing hysteresis of superconducting ceramic samples with great number of interfaces has been established. The dependences of residual magnetization on magnetizing field and temperature were measured and the effective flux pinning force was extracted from the magnetizations according to the method mentioned above. The force consisted of two types, one of which results from interface magnetic currents which conduct the force to crystal lattice directly, and governed by weak links if there are. The other arises from the flux pinning of intragrain pinning centres. The former is important in small fields whereas the later is dominant in high fields.