Global warming has become one of the important social problems. The main cause for the global warming is carbon dioxide, CO₂ generated when fossil fuels such as oil and natural gas are burned. Thereby recently the momentum for reducing CO₂ emissions in maritime industry has been growing globally. International Maritime Organization, IMO, agreed a GHG reduction target in 2018. A long-term goal is to achieve zero GHG emissions from ships as early as possible in this century. In order to realize a zero GHG emission ship, it is needed to use zero-carbon fuel such as hydrogen and ammonia. In this paper, the author shows various alternative fuels and the technologies for the utilization, and discusses the technical issues for realizing the zero-emission ship.