Safe voyages of ships have been threatened by piracy and armed robbery against ships in particular in these years. Under such circumstances, ships have employed various countermeasures against piracy. On the other hand, the effectiveness of various countermeasures against piracy has not yet been evaluated quantitatively. In this paper, the authors analyze the effectiveness of various countermeasures against piracy based on the incident data contained in “reports on acts of piracy and armed robbery against ships” issued by the International Maritime Organization. From incident data since 2008, protective measures taken onboard have been derived and the relations between the protective measures and severities of consequences are analyzed.