Autonomous ships, including remotely controlled ships, are expected to have positive effects, such as decreasing the number of marine accidents caused by human errors or improving the working conditions of seafarers. However, relevant laws and regulations must be updated to fully deploy this new technology, as the existing maritime legal framework does not expect autonomous ships. This article describes and analyzes the discussions at the International Maritime Organization (IMO) on maritime autonomous surface ships (MASS). 　Legal issues regarding autonomous ships can be summarized into three categories. First, IMO conventions that introduce safety regulations for ordinary ships (e.g., SOLAS, COLREG, or STCW) must be amended or supplemented to accommodate autonomous ships. While the IMO’s Maritime Safety Committee is now drafting the MASS Code to deal with the technical aspects of this issue, several items, such as the roles of the master of MASS, are also discussed in the MASS Joint Working Group. 　Second, it is necessary to confirm that autonomous ships are compatible with the United Nations Convention on the Law of the Seas, which forms the foundation of the regulations by the IMO. 　The third issue is the legal liability for accidents involving autonomous ships, particularly the civil liability of the shipowners and other interested parties. Despite its importance, however, this issue has been underexplored in the IMO so far, as no existing IMO convention covers the liability of shipowners for ships in general.