An outbreak of amikacin- and ciprofloxacin-resistant Acinetobacter baumannii ST219 in Tokai University hospital's emergency intensive care unit was caused by its colonization in water systems and subsequent spread through oral care using tap water. The outbreak was successfully controlled after replacement of the water system and implementation as of daily cleaning of water taps and oral care with a dry method. It is important to strictly manage the water system in critical care areas.