According to the invention, an injection device (D) comprises a pre-filled syringe and a safety device. The pre-filled syringe comprises a barrel with an inner cavity in fluid communication with a hypodermic needle attached to a distal end of the pre-filled syringe and a piston fluid-tightly sealing a proximal end of the inner cavity. The safety device comprises a support body for retaining the pre-filled syringe therein, an outer body operably connected to the piston, a needle shield and a releasable retaining means (R). The retaining means comprises a radial projection and a longitudinal recess adapted to receive the radial projection. The radial projection is connected to or integral part of one of the support body or the outer body and a longitudinal recess formed into the other of the support body or the outer body. The needle shield, the support body and the outer body are telescopable relative to each other. The piston is movable in a distal direction to expel a medicament contained in the inner cavity through the hypodermic needle by a manual actuation of the outer body. A telescoping movement of the outer body with respect to the support body is restricted by the retaining means (R) when the needle shield is moved with respect to the support body in a proximal direction from an initial position (I) towards a retracted position (II). The retaining means (R) is released when the needle shield reaches the retracted position (II) allowing for a telescoping movement of the outer body with respect to the support body to expel the medicament contained in the pre-filled syringe through the hypodermic needle.