To provide a lock mechanism for a string of an electric guitar which can prevent the string from becoming out of tune or shifting in position during locking for clamping the string with a clamp pad by increasing the pressing force and contact frictional resistance of the string to a nut and facilitates mounting operation for the string.

The nut 21 is fixed to the top surface of a neck 13. The clamp pad 22 is clamped and fixed to a pinch surface 21a of the nut 21 with a clamp bolt 23 to pinch the string 15 with the pinch surface 21a. The tilt angle β of the straight line L3 connecting a 1st critical contact point P1 and a lock point P4 of the string to the straight line L2 connecting 1st and 3rd critical contact points P1 and P3 at both ends of the pinch surface 21a is set to 2 to 12.5°. Then the contact frictional force of the string 15 at a 3rd critical contact point P3 on the pinch surface 21a is increased to prevent the string 15 from becoming out of tune or shifting in position during the locking.