Abstract: A device for preventing injury to the fingers of a person holding a cap into which an injector needle is inserted after use. The device is formed as a plate shaped member having a plane substantially transverse to the axis of the tube and having an area large enough to stop movement of the needle is not properly inserted into the cap when capping the injector. The plate member is provided with a thin-walled portion having a central opening and/or axial slits having inner edges which are elastically flexed and thus maintan a tight grip on the outer surface of the cap. Accordingly, the plate member is firmly fixed to the cap, and prevents injury to the fingers of a person holding the cap, by the needle, of the injector.