Disclosed is a disposable blood collecting instrument that stably secures a sufficient volume of blood with a single puncture. A tip portion (63) of a piercing body (62) of a lancet (11) contained within a housing (10) projects by means of a biasing force of a spring assembly (12), and a surrounding area of an opening (38) of the housing (10) is made a transparent visible portion (59) with a visible interior. Additionally, the disposable blood collecting instrument has been formed such that the spring assembly (12) is contained within housing farther behind the piercing body (62) than a position of the tip portion (63) of the piercing body (62) that has entered within the housing (10) after piercing.
