Defective system or human error in health care organizations often causes medical errors. While the minor damage is negligible, fatality of patient will be caused by a severe mistake. The patient safety has recently received a great deal of attention. Every accident is accumulated from a lot of small errors. People are used to these small errors, because it does no harm. However, when these small mistakes take place together, a disaster happens. The Swiss cheese model explains system accidents. To provide a safe patient care, health care organization needs to establish an intact system and operate the procedure strictly and monitor frequently.