Back-rest structure for automobile vehicle seat comprising: • A plastic ring ∘ comprising four parts, a first lateral part (3.1), a second lateral part (3.2), a third upper part (3.3) that connects said first and second lateral parts (3.1, 3.2) at the top, and a fourth lower part (3.4) that connects said first and second lateral parts (3.1, 3.2) at the bottom, • A U-shaped reinforcement frame made of a metal sheet, open on the bottom: ∘ that comprises three parts a first lateral part (2.1), a second lateral part (2.2), and a third upper part (2.3) that connects said first and second lateral parts (2.1, 2.2), ∘ has a U-shaped section wherein the open face (14.1) of said U-shaped section corresponding to the first lateral part (2.1) and the open face (14.1) of the U-shaped section corresponding to the second lateral part (2.2) are opposite each other, ∘ wherein the thickness of the metal sheet forming said reinforcement frame is less than 1.5 mm, • first joining means that lock the position of the ring with respect to the reinforcement frame in a mounting direction m for both elements, wherein said ring is fitted in the reinforcement frame such that the seat back-rest structure has several contacting sections in correspondence with the first, second and third parts (2.1, 2.2, 2.3) of the reinforcement frame and in correspondence with the first, second and third parts (3.1, 3.2, 3.3) of the ring, respectively, such that each contacting section comprises contacting areas formed by part of the surface of the ring arranged without clearance in correspondence with a part of the surface of the reinforcement frame, thereby allowing loads to be transmitted between said ring and said reinforcement frame, and wherein the assembly formed by said ring and said reinforcement frame joined by the first joining means form a single structure that can withstand the loads to which the seat back-rest is subjected in the direction x of the vehicle. • A plastic ring ∘ comprising four parts, a first lateral part (3.1), a second lateral part (3.2), a third upper part (3.3) that connects said first and second lateral parts (3.1, 3.2) at the top, and a fourth lower part (3.4) that connects said first and second lateral parts (3.1, 3.2) at the bottom, ∘ comprising four parts, a first lateral part (3.1), a second lateral part (3.2), a third upper part (3.3) that connects said first and second lateral parts (3.1, 3.2) at the top, and a fourth lower part (3.4) that connects said first and second lateral parts (3.1, 3.2) at the bottom, • A U-shaped reinforcement frame made of a metal sheet, open on the bottom: ∘ that comprises three parts a first lateral part (2.1), a second lateral part (2.2), and a third upper part (2.3) that connects said first and second lateral parts (2.1, 2.2), ∘ has a U-shaped section wherein the open face (14.1) of said U-shaped section corresponding to the first lateral part (2.1) and the open face (14.1) of the U-shaped section corresponding to the second lateral part (2.2) are opposite each other, ∘ wherein the thickness of the metal sheet forming said reinforcement frame is less than 1.5 mm, ∘ that comprises three parts a first lateral part (2.1), a second lateral part (2.2), and a third upper part (2.3) that connects said first and second lateral parts (2.1, 2.2), ∘ has a U-shaped section wherein the open face (14.1) of said U-shaped section corresponding to the first lateral part (2.1) and the open face (14.1) of the U-shaped section corresponding to the second lateral part (2.2) are opposite each other, ∘ wherein the thickness of the metal sheet forming said reinforcement frame is less than 1.5 mm, • first joining means that lock the position of the ring with respect to the reinforcement frame in a mounting direction m for both elements,