Abstract: A self-injection device has a tubular sleeve which is open at its posterior end and partially closed at its anterior end. An ampule sealed at its posterior end by a piston and at its anterior end by a membrane is axially arranged within the tubular sleeve. An injection needle having a terminal flange at its posterior end and an anterior end for piercing the membrane is axially arranged in the ampule. A tubular actuating cap having a piston rod emanating from the bottom of cap for acting upon the piston is slidably guided on the sleeve. The actuating cap and the piston rod are traversed by a central bore having axially extending grooves. The device also includes a locking pin having radially projecting spring tongues. A first segment of the spring tongue extends in an axial direction on the periphery of the locking pin and a second segment is bent at an obtuse angle, extending obliquely outward in a radial-axial direction through the axially extending grooves in the central bore.