A device (1; 101) for injection of a product into an injection site, said device comprising: a housing (40; 140) having a longitudinal axis A and receiving a container for the product to be injected, said container being aligned on said longitudinal axis, said container being substantially closed at a proximal end by a stopper, said stopper being capable of being moved distally within said container so as to expel the product to be injected, and at a distal end by a needle for the exit of the product to be injected, said container being movable with respect to said housing between a first position, in which the needle does not extend beyond a distal end of the housing, and a second position, distally spaced with respect to said first position, in which the needle extends beyond the distal end of the housing, biasing means (8; 108), coupled to said container and to said housing at least from said first position to said second position of the container, designed for exerting a distal force on said container so as to move said container from its first position to its second position when going from a first state to a second state, said second state being less stressed than said first state, retaining means (70; 170) coupled to said container and to said housing in the first position of the container, for releasably maintaining said biasing means in its first state, said retaining means being capable of moving from a passive condition, in which it maintains said biasing means in its first state, to an active condition, in which said biasing means is free to expand to its second state, triggering means (90; 190) capable of moving said retaining means from its passive condition to its active condition, characterized in that said retaining means comprise a lever member (70; 170) having a pivoting part (71; 171) rotatable around an axis R1 and at least a radial projection (73; 172) extending from said pivoting part, said radial projection being in a first angular position when said retaining means is in its passive condition, said radial projection being in a second angular position, different from said first angular position, when said retaining means is in its active condition, said axis R1 of said pivoting part being included in a transversal plane of said longitudinal axis A, wherein, in its first angular position, the radial projection (73; 172) extends in an oblique or transversal direction with respect to the distal direction, and in its second angular position, the radial projection extends in the distal direction. a housing (40; 140) having a longitudinal axis A and receiving a container for the product to be injected, said container being aligned on said longitudinal axis, said container being substantially closed at a proximal end by a stopper, said stopper being capable of being moved distally within said container so as to expel the product to be injected, and at a distal end by a needle for the exit of the product to be injected, said container being movable with respect to said housing between a first position, in which the needle does not extend beyond a distal end of the housing, and a second position, distally spaced with respect to said first position, in which the needle extends beyond the distal end of the housing, biasing means (8; 108), coupled to said container and to said housing at least from said first position to said second position of the container, designed for exerting a distal force on said container so as to move said container from its first position to its second position when going from a first state to a second state, said second state being less stressed than said first state, retaining means (70; 170) coupled to said container and to said housing in the first position of the container, for releasably maintaining said biasing means in its first state, said retaining means being capable of moving from a passive condition, in which it maintains said biasing means in its first state, to an active condition, in which said biasing means is free to expand to its second state, triggering means (90; 190) capable of moving said retaining means from its passive condition to its active condition, characterized in that said retaining means comprise a lever member (70; 170) having a pivoting part (71; 171) rotatable around an axis R1 and at least a radial projection (73; 172) extending from said pivoting part, said radial projection being in a first angular position when said retaining means is in its passive condition, said radial projection being in a second angular position, different from said first angular position, when said retaining means is in its active condition, said axis R1 of said pivoting part being included in a transversal plane of said longitudinal axis A, wherein, in its first angular position, the radial projection (73; 172) extends in an oblique or transversal direction with respect to the distal direction, and in its second angular position, the radial projection extends in the distal direction.