The present invention relates to an injection assistance device (1) for an injection device (3) comprising one body (2), one needle (7), and one piston plunger (26), characterized in that it comprises: a hollow sleeve (4) receiving the body (2), coupling arrangement (10) for moving the piston plunger (26) from an end-of-insertion position to an end of injection position the coupling arrangement being maintained in a passive state until the end-of-insertion position, spacer arrangement (8) to maintain the piston plunger (26) and the body (2) in an initial position, disengaging arrangement (22, 14, 15, 16) linked with the spacer arrangement (8), the spacer arrangement (8) and/or disengaging arrangement (22, 14, 15, 16 25, 31) being arranged in order not to free the distal displacement of the piston plunger (26) before the one of the body (2).
