Hagi ware originally consists of a mixture of two raw materials: Daido clay and Mishima clay. During its firing process, we observed a change in the magnetic properties of the iron oxide, Fe 2 O 3 . The magnetic moment of the Daido clay (which only contains a small amount of the Fe 2 O 3 £-phase) attains a maximum at a firing temperature of approximately 600°C, where a minor amount of the poorly crystallized Fe 2 O 3 temporarily changes to the ferromagnetic £-phase. Furthermore, the magnetic moment of the Mishima clay (which contains a large amount of the Fe 2 O 3 £-phase) decreases as the firing temperature increases, whereas the coercive field rapidly increases at firing temperatures above 1000°C. The magnetization curve of the Mishima clay that was fired at temperatures above 1200°C is characteristic of a two-component system consisting of a minor £-phase and a major ¡-phase. The above-mentioned phenomena were also confirmed by XRD analyses. A series of experiments indicated that the firing of Hagi ware can be characterized as a transformation from the £-phase of Fe 2 O 3 to the ¡-phase of Fe 2 O 3 . This transformation is considered to contribute to the change from soft magnetism to hard magnetism of Hagi ware.