The sustainability of maritime trade and the marine environment is related to the safe management and operation of ships. Thus, ISM Code was enforced to ensure the safety management and operation of ships and prevent sea pollution. This study seeks to reveal the importance levels of the factors determining the effectiveness of the ISM Code and to identify their causal relationships. Through the fuzzy DEMATEL method to examine the data obtained from safety and quality experts, and seafarers, it aims to analyze whether the effectiveness of the ISM Code differs by ship types and land-based personnel and seafarers. All participants consider “management’s efforts to strengthen the ISM system” (C2) as the most important factor. Moreover, this study reveals that the factors of “management’s safety priority and commitment” (C1) and (C2) that determine the effectiveness of the ISM system are in the group of affecting factors. Identifying the factors increasing the effectiveness of the ISM Code, and revealing their causal relationships, this study adds to the literature. This study is important as it explores the need of managements to prioritize safety, to strengthen commitment and systems for an effective ISM Code, ensuring safe management of ships, and prevention of sea pollution.