Fastening device for fastening an assembly comprising at least two planar elements , the planar elements being provided with openings that face each other when the planar elements are in the assembly position, the fastening device comprising: a. a clip which is intended to be inserted into the openings of the planar elements and intended to hold the planar elements together between a first support point (A) and a second support point (B), the clip comprising: i. a planar head for forming the first support point (A) on the main surface of one of the planar elements , the planar head having a central opening in order to access an inner cavity of the clip ; ii. two lateral upright members designed to form the second support point (B) on another of the planar elements , the lateral upright members having a first end which is secured to the head as well as a second end, the second ends being connected to each other in such a way that the two lateral upright members define the inner cavity of the clip ; b. a pin which is intended to be inserted into the inner cavity of the clip in order to lock the assembly; the fastening device being characterized in that the pin comprises at least one flexible bridging piece having a central portion which is connected to a main body of the pin by two flexible portions , the flexible bridging piece being intended to come into contact with a lateral upright member of the clip in order to force the second support point (B) and to hold the assembled planar elements together. a. a clip which is intended to be inserted into the openings of the planar elements and intended to hold the planar elements together between a first support point (A) and a second support point (B), the clip comprising: i. a planar head for forming the first support point (A) on the main surface of one of the planar elements , the planar head having a central opening in order to access an inner cavity of the clip ; ii. two lateral upright members designed to form the second support point (B) on another of the planar elements , the lateral upright members having a first end which is secured to the head as well as a second end, the second ends being connected to each other in such a way that the two lateral upright members define the inner cavity of the clip ; i. a planar head for forming the first support point (A) on the main surface of one of the planar elements , the planar head having a central opening in order to access an inner cavity of the clip ; ii. two lateral upright members designed to form the second support point (B) on another of the planar elements , the lateral upright members having a first end which is secured to the head as well as a second end, the second ends being connected to each other in such a way that the two lateral upright members define the inner cavity of the clip ; b. a pin which is intended to be inserted into the inner cavity of the clip in order to lock the assembly; the fastening device being characterized in that the pin comprises at least one flexible bridging piece having a central portion which is connected to a main body of the pin by two flexible portions , the flexible bridging piece being intended to come into contact with a lateral upright member of the clip in order to force the second support point (B) and to hold the assembled planar elements together. <EOS>