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. When fully inserted into the device, the spring tongues contact the posterior end of the sleeve to prevent unintentional triggering of the injection needle.