to bias it this surface 17 extends transversely below the second wall portion 16 b of the adjustment extension 11 a 11 b it is noted that the assembly of the extensions 11 a 11 b on the main band portion 8 due to attaching arrangement 15 can occur in a plurality of positions as seen in fig4 the slot 15 of the extensions is capable of cooperating with a plurality of positioning pins 18 of the band 8 thus this plurality of possible assemblies multiplies the possibilities for adjusting the size of the headband 5 by being advantageously combined with the adjusting device 6 of the invention it is understood that one would not leave the scope of the invention by making the band and its adjustment extensions in one single piece according to the preferred embodiment of the helmet 1 of the invention the locking buckles 13 adapted to cooperate with the extensions 11 a 11 b are borne by or coupled to a rear connecting element 12 that partially forms the headband 5 the headband 5 is arranged in the helmet so that the connecting element 12 is arranged centrally at the rear of the helmet 1 substantially at the level of the user 39 s nape this connecting element advantageously has two guiding slots 19 adapted to cooperate with a guiding pin 20 for each of the.