PURPOSE: To improve the musical performance without spoiling the acoustic effect by eliminating the pressure of the right arm elbow when the acoustic guitar which has a large resonance body is played.

CONSTITUTION: The front surface edge of the side plate 3 which forms the large-diameter part of the resonance body 1 is cut over a certain range; and a cut groove 8 is formed in the internal surface of the top plate 2, which is folded inward along the cat groove 8 to form a slanting surface part 9. The peripheral edge of the slanting surface part 9 is joined with the cut front surface edge of the side plate 3 and a bellows-shaped batten 10 is stuck on the joint part from inside, a force wood piece 14 cut in conformity with its folding shape is stuck crossing the cut groove 8 in the inside surface of the top plate 2, and the tip 14' of the force wood piece 14 is engaged with the batten 10.