Comparing with detention, the purpose of preventive detention, however, is not for ensuring the proceeding of litigation procedure, but for a preventive measure and which is in similarity with a security measure. Thus, the conditions between preventive detention and detention are very different. Against the substance and the purpose of detention which lays in crime investigation, preventive detention violates the presumption of innocence with the basis of due process of law. Nevertheless, as the importance of precaution in any dangerous behaviors to society, preventive detention is still a mainly purpose of detention in many countries nowadays. In different aspects practically, although there are vary opinions and argues in the rationality of preventive detention, preventive detention is still recognized to society resulted from the identification of safeguard to the public. The thesis researches the reasonableness of preventive detention, the social demands of preventive detention, and the influences over criminal policy under the social demands of preventive detention. Presumption of innocence, due process of law and the human rights are the core issues about preventive detention; however, the influences of preventive detention over the criminal policy are much obvious under the studies of risk society. The thesis states the background of the legal progress in Taiwan for analyzing the system of preventive detention in Taiwan, and also researches into the risk society theory in western society for the possibility of defining the same in the system of preventive detention in Taiwan. The thesis holds a dissent opinion from the existence of preventive detention. Nevertheless, as it has difficulties to abolish the preventive detention system in whole under the practical legislation, the system of preventive detention shall be amended in the conditions of acceptance by society.