Abstract: An object of the invention is to achieve an increased speed of blood collection and separation as well as ease of handling of plasma components after blood separation. A blood-collecting tube, which draws in blood and separates it by using an internal negative pressure in the tube, comprises an upstream tube 10 having a blood inlet 11 and blood outlets 18, a filter 20 for separating blood and a downstream tube 12. A plug 18 is fitted to the blood inlet 11 to thereby close the interior of the upstream tube 10, and the filter 20 is installed so that it blocks the blood outlets 18. The downstream tube 12 is disposed to enclose the filter 20 and constructed in such a way that the downstream tube 12 can be attached to and detached from the upstream tube 10.