Injection device of a product comprising a storing device and a needle assembly to be used in combination with the storing device, said storing device comprising at least: - a barrel of substantially tubular shape having a distal end and a proximal end, said barrel defining a chamber for receiving said product, - a piston intended to expel said product through the distal end under a distal force exerted on said piston, said storing device being prefillable with said product and said piston sealing the proximal end of said barrel, said needle assembly comprising at least: - a needle-bearing portion, designed for receiving a needle and movable between an exposed position in which said needle is exposed and a retracted position in which the needle is hidden in said barrel, the said needle-bearing portion being bom by a needle hub, said needle-hub and needle-bearing portion being made of a first material, - engaging means (8a, 10, 11, 18, 19, 20, 23), designed for securing said needle-bearing portion to said distal end of said barrel, at least in the exposed position, characterized in that: said needle assembly comprises: - sealing means designed for tightly sealing the distal end of said chamber to said needle-bearing portion so as to avoid any leakage of said injectable product from the distal end of said chamber to the needle-bearing portion, at least in the exposed position, and reduce product permeation, said sealing means being made of a second material, - a breakable portion located between said engaging means (8a, 10, 11, 18, 19, 20, 23) and said needle-bearing portion, said breakable portion being arranged to, when broken, allow independent proximal displacement of said needle-bearing portion relative to said engaging means (8a, 10, 11, 18, 19, 20, 23) between said exposed and said retracted positions, - said first material being more rigid than said second material, and - said injection device further comprises driven means for causing said piston to move toward the distal end of said needle assembly and realize the injection of the product, said driven means comprising a hollow plunger rod designed to receive at least said needle in its retracted position. - a barrel of substantially tubular shape having a distal end and a proximal end, said barrel defining a chamber for receiving said product, - a piston intended to expel said product through the distal end under a distal force exerted on said piston, said storing device being prefillable with said product and said piston sealing the proximal end of said barrel, - a needle-bearing portion, designed for receiving a needle and movable between an exposed position in which said needle is exposed and a retracted position in which the needle is hidden in said barrel, the said needle-bearing portion being bom by a needle hub, said needle-hub and needle-bearing portion being made of a first material, - engaging means (8a, 10, 11, 18, 19, 20, 23), designed for securing said needle-bearing portion to said distal end of said barrel, at least in the exposed position, said needle assembly comprises: - sealing means designed for tightly sealing the distal end of said chamber to said needle-bearing portion so as to avoid any leakage of said injectable product from the distal end of said chamber to the needle-bearing portion, at least in the exposed position, and reduce product permeation, said sealing means being made of a second material, - a breakable portion located between said engaging means (8a, 10, 11, 18, 19, 20, 23) and said needle-bearing portion, said breakable portion being arranged to, when broken, allow independent proximal displacement of said needle-bearing portion relative to said engaging means (8a, 10, 11, 18, 19, 20, 23) between said exposed and said retracted positions, - said first material being more rigid than said second material, and - said injection device further comprises driven means for causing said piston to move toward the distal end of said needle assembly and realize the injection of the product, said driven means comprising a hollow plunger rod designed to receive at least said needle in its retracted position. - sealing means designed for tightly sealing the distal end of said chamber to said needle-bearing portion so as to avoid any leakage of said injectable product from the distal end of said chamber to the needle-bearing portion, at least in the exposed position, and reduce product permeation, said sealing means being made of a second material, - a breakable portion located between said engaging means (8a, 10, 11, 18, 19, 20, 23) and said needle-bearing portion, said breakable portion being arranged to, when broken, allow independent proximal displacement of said needle-bearing portion relative to said engaging means (8a, 10, 11, 18, 19, 20, 23) between said exposed and said retracted positions, - said first material being more rigid than said second material, and - said injection device further comprises driven means for causing said piston to move toward the distal end of said needle assembly and realize the injection of the product, said driven means comprising a hollow plunger rod designed to receive at least said needle in its retracted position.