A tightening string is used in an endoscope for firmly securing an outer cover of a flexible tube of the endoscope onto an internal member disposed inside the outer cover. The tightening string is comprised of a filament assembly formed by bundling a plurality of filaments made of a synthetic resin. The invention also provides an outer cover securing method, a flexible tube for an endoscope, and an endoscope equipped with the flexible tube. An outer cover securing method using the tightening string, a flexible tube for an endoscope equipped with the thus secured outer cover, and an endoscope equipped with the flexible tube are also provided.
