PURPOSE: To provide a method and a device that can perform accurate and optimum positioning of a guide rod at the center of a play between the guide rod and a broach in relation to positioning (centering) by inserting the guide rod coaxial with the broach including a machining tool in a valve guide, in machining the valve seat of an engine.

CONSTITUTION: A guide rod 1 can move along the peripheral edge of a valve guide, and a broach 15 is turnably provided in a pivoting ball 5. The pivoting ball 5 can rotate and turn in all directions and is provided with a follower lever 8 energized by a spring in its external periphery with the two tip portions (stoppers) of the follower lever 8 facing each other. The guide rod 1 sweeps all around the valve guide, drawing a rectangular or rhomb on a plane at first due to the nutation of the guide rod, but as the nutation progresses, the pivoting ball 5 is placed at the center of the swept shape, moving the follower lever and centering lever 22, where it is finally fixed by means of a clamping bridge 23.