A plug that allows the plug body to be easily and reliably pierced through by a hollow bodily fluid collection needle, makes the bodily fluid collection needle difficult to draw back out of the plug body owing to a resilience of a sheath after the start of bodily fluid collection, and enables easy and reliable collection of the bodily fluid. The plug 1 is a plug for closing an opening end of a bodily fluid-collecting container 14 which is to be inserted into a tubular holder 11, and includes: a plug body 2 to be fixed to the opening end of the bodily fluid-collecting container 14 and a cover member 3 sheathed on the plug body 2 to cover an outer surface of the plug body 2, wherein the cover member 3 includes a side part 3c covering the outer surface of the plug body 2, and an outer surface of the side part 3c is provided with a protrusion 3k which protrudes radially outward of the side part 3c of the cover member 3 to come into contact with and give frictional resistance to an inner wall of the holder 11 upon insertion into the holder 11.
