To position a prepared hole accurately on every crankshaft with the dispersion of a prepared hole within tolerance in premachining by providing a positioning pin with a centering means for adjusting the center of the prepared hole to a machining axis.

In case the diameter of a positioning pin 8 is smaller than a prepared hole (h), a pair of ball plungers 13, 13 come in contact with the internal wall of the prepared hole (h) to make spring energizing force the same, so that the center of the prepared hole (h) can coincide accurately with the axial center of the positioning pin 8. The axis of the positioning pin is previously made coincide with the axis of a machining tool. A temporary receiving part is retreated to pull the positioning pin 8 from the prepared hole (h), and at the same time, the machining tool is advanced from an opposite direction to machine the prepared hole (h). Centering is therefore performed correctly, so that machining can be performed with accuracy.