Mandibular advancement device (1 20) and a device preparation method and kit, comprising a mouthpiece (2) and at least one plastic strap (5 21) with two ends (7) fitted with a slot (9) that is connected articulately to a respective cylindrical portion (11) of a lateral protrusion (10) of the mouthpiece (2). The slot (9) is closed and is longer in length than the diameter of the cylindrical portion (11), which can move along the slot (9). The strap (5 21) acts by traction between the cylindrical portions (11) of the mouthpiece (2) to prevent the free return of the lower portion (4) with respect to the upper portion (3) of the mouthpiece (2) and therefore treats apnea, whilst allowing some advancement and lateral movement of the lower portion (4), making it comfortable for the user.
