KEKB stopped the operation on June 30, 2010, and has started to upgrade to SuperKEKB aiming at 40 times higher luminosity to search for new physics beyond the Standard Model. Since 2001 KEKB has been running at the world highest luminosity and has achieved the record of 2.11×1034 cm-2s-1, which is more than twice the design luminosity. This paper describes the history, achievements, and operation experiences of KEKB.