This paper describes a sliding-mode observer design for state estimation of a motorized seat belt (MSB) system. The MSB system is an active safety system that can protect passengers and improve passenger convenience by several operations, which prevent and warn to passengers about dangerous driving conditions and reduce the dangerous situation caused by slack and poor return of seat belt. To realize these operations, state information such as the belt winding velocity and tension of belt are required. However measuring this information with sensors increases the system cost and complexity. In this paper, a sliding-mode observer, which estimates requisite state information, is proposed. The designed sliding-mode observer is robust towards system uncertainties and parameter variation and is verified in matched and mismatched parameter cases.