Since its establishment, the B-factory at KEK has been operating successfully, achieving successive world records for the luminosity of electron-positron colliders. The factory has also conducted experimental tests related to the Kobayashi-Maskawa theory. First of all, large CP violation was discovered in the B meson system, which is one of the predictions of the model. This was followed by confirmation in various B meson decays. On the other hand, some phenomena have been found which may not be fully explained by the model alone. Expectations are high for clarification using upgraded B-factory(ies).