Injection assistance device comprising an injection device for injecting a product into an injection site, this injection device comprising at least one hollow body intended to receive a product that is to be injected, at least one hollow injection needle intended to penetrate the injection site, and at least one piston plunger housed in the said body, the said body and the said piston plunger being able to be moved in axial translation one with respect to the other, characterized in that the said injection assistance device comprises at least: - a hollow sleeve provided with at least one bearing surface intended to come into contact with the surface of the said injection site, the said sleeve receiving, at least in part, the said body and being arranged in such a way as to allow the said body axial mobility between at least a first position known as the initial position in which the said needle is not exposed over its insertion length, and a second position known as the insertion position in which the said needle is exposed by a predetermined insertion length L, - a plunger rod capable of being coupled to said piston plunger at least during an injection step for moving said piston plunger from an end-of-insertion position to an end of injection position and to administer the injection when submitted to a distal pressure exerted on them, - spacer means for preventing movement of the plunger rod relative to said body in the insertion position, - disengaging means (60, 63) linked with said spacer means, said spacer means and/or disengaging means (60, 63) being arranged in order not to free the distal displacement of said piston plunger before the one of said body, - automatic-injection means for urging the said plunger rod at the end of the insertion position without manual intervention on the part of the user, - maintaining means for keeping the said plunger rod in the said insertion position, the said automatic-injection means being activated by the release of the said maintaining means and - with the said body comprising a flange at its proximal end, the said maintaining means comprise at least one deflecting tab engaging the said body and the said plunger rod in the insertion position, the said tab being able to deflect and to disengage the said body from the said plunger rod by collaborating with the said flange at the end of the insertion position, said assistance injection device further comprising: automatic-insertion means for causing the said body to move axially in the distal direction and to insert the said hollow needle into the injection site, - retaining means (61, 62) for retaining the said body in the said initial position, the said automatic-insertion means being activated by the release of the said retaining means (61, 62). - a hollow sleeve provided with at least one bearing surface intended to come into contact with the surface of the said injection site, the said sleeve receiving, at least in part, the said body and being arranged in such a way as to allow the said body axial mobility between at least a first position known as the initial position in which the said needle is not exposed over its insertion length, and a second position known as the insertion position in which the said needle is exposed by a predetermined insertion length L, - a plunger rod capable of being coupled to said piston plunger at least during an injection step for moving said piston plunger from an end-of-insertion position to an end of injection position and to administer the injection when submitted to a distal pressure exerted on them, - spacer means for preventing movement of the plunger rod relative to said body in the insertion position, - disengaging means (60, 63) linked with said spacer means, - automatic-injection means for urging the said plunger rod at the end of the insertion position without manual intervention on the part of the user, - maintaining means for keeping the said plunger rod in the said insertion position, the said automatic-injection means being activated by the release of the said maintaining means and - with the said body comprising a flange at its proximal end, the said maintaining means comprise at least one deflecting tab engaging the said body and the said plunger rod in the insertion position, the said tab being able to deflect and to disengage the said body from the said plunger rod by collaborating with the said flange at the end of the insertion position, automatic-insertion means for causing the said body to move axially in the distal direction and to insert the said hollow needle into the injection site, - retaining means (61, 62) for retaining the said body in the said initial position, the said automatic-insertion means being activated by the release of the said retaining means (61, 62).