The mechanism includes: an urging means for urging an inner needle (12) to the side opposite to an outer needle (11) with respect to the axial direction of a holder sleeve (1) for holding the inner needle (12); an actuator (6) which is positioned between the urging means (8) and inner needle (12), moves together with the inner needle (12) and has a puncture position retainer for keeping the inner needle (12) and outer needle (11) ready for puncture and an inner needle retraction actuating portion for allowing for the retraction actuation of the inner needle (12); and a slit (2) for assuring the path of movement of the actuator (6). The puncture position retainer has a puncture position engagement portion whereby the actuator (6) is engaged with the holder sleeve (1) at a position on the outer needle side (11). The inner needle retraction actuating portion includes: an actuating portion (6a) for releasing the actuator (6) from the engaged state of the puncture position engagement portion into the urged state by the urging means (8) and an actuator housing for enclosing the actuator after retraction of the inner needle.