According to a blood plasma collection tool of the present invention, a phenomenon that blood cells in blood spontaneously precipitate due to an effect of gravitational force takes place in the very narrow microspace (separation part) having the very narrow depth in the direction of gravitational force of not greater than 1 mm, and the overflow channel functions as a dam against the separation part so that the blood plasma separated out as supernatant fluid can overflow beyond the overflow channel, whereby the blood cells separated out can be prevented from entering the collection part. Therefore, blood plasma can be collected by accurately and readily separating blood plasma and blood cells in a small amount of blood from each other in a short time, using a small tool having a simple structure.