Abstract: A belt tightener includes, in addition to a clasping mechanism for a belt to hold motor vehicles and other heavy loads firmly in a given position of transportation device such as a carrier vessel, a specific device for preventing the belt from loosening during its actual use under shaking conditions. The belt tightener can thus, be used advantageously to firmly tie down motor vehicles, cargo containers, heavy objects, and the like with a belt to fixing structures on carrier vessels while preventing the belt from loosening by external forces exerted during rough seas when the vessels are rolling and pitching.