The present report describes myofunctional therapy using a ready-made training device, the T4A, in patients with permanent dentition and its effect on the prevention of relapse. The buccinator mechanism maintains the inner pressure of the tongue muscle equivalent to the outer pressure of the perioral soft tissues, such as the orbicular muscles, including the cephalopharyngeus and buccinator muscles. Training is performed so that patients learn to place their tongue and lips in the appropriate resting positions. The shape of the T4A and tongue guard supports the tongue from the bottom, allowing formation of the correct resting tongue position. However, the use of T4A for a long period of time may cause the teeth movement; therefore, caution is required. Use of the T4A is effective for the correction of oral habits, myofunctional therapy and for teaching the correct resting tongue position during the daytime and for the correction of oral habits, teaching correct resting tongue position during sleep. Schlagwörter: myofunctional therapy, orthodontic treatment, ready-made mouthpiece