To provide a boring method of an NC machine tool capable of preventing any deviation between the axis of a prepared hole and the axis of a finished hole, and preventing any left-over of the prepared hole.

In the boring method of the NC machine tool 1, a prepared hole is formed by a rotary blade movable toward the target position corresponding to the machining position of a workpiece which is held to a chuck body 10, and a finished hole H2 of the predetermined dimension is formed by the cutting process by turning to the prepared hole by a turning tool 26 while rotating the chuck body with a spindle as a center of rotation. The boring method comprises a first step of calculating the axial deviation L between the axis of the prepared hole formed in a first workpiece W1 by the rotary blade moved to the first target position and the axis of the finished hole formed in the first workpiece, and a second step of forming a prepared hole in a second workpiece W by moving the rotary blade to the second target position with the axial deviation calculated by the first step being added to the first target position.