A device for introducing a catheter into a body site through the skin by means of an security non-piercing cage includes a steel spring flexible blade which is disposed in the cross section of the chamber of the nonpiercing cage near the proximal input thereof perpendicularly to a needle and is penetrable by the needle. The blade and the needle are adapted to interact in such a way that the blade is in a resting position and freely transversable by the needle when the needle is pushed in a distal direction, and the blade stops the needle and is flexed thereby when the needle is pulled in a proximal direction beyond a determined axial position in such a way that the flexed blade tilts the needle and exposes the inclined needle to a restoring force which pushes the needle off in the distal direction until the puncture end is stopped against the chamber wall. The invention can be used for intravenous catheters.