PURPOSE: To improve decentering correction accuracy with mandays required for correcting decentering reduced, by forming a marking line on a position at an equal distance from the small hole of an axis display part, and arranging the surface of a boring bar on a position at an equal distance from the marking line.

CONSTITUTION: A linear laser beam B is passed through along an axis position in design of a stern tube 1 from a laser beam source 20 located outside the stern tube 1. The axis position adjusting mechanism of an axis display member 9 is adjusted so that the laser beam B can be passed through a small hole formed on the axis display part 10 of an axis display member 9. Next, a circular marking line, having the same radius, is formed on a protruded member 8, formed on the end surface of the stern tube 1 and a run off part 3, by a compass 21, having a needle engaged with the small hole, with the small hole of the axis display part 10 as a center. Then a boring bar 4 is inserted in the stern tube 1 after the axis display member 9 is removed, and the surface of the boring bar 4 is located on a position having an equal distance from the marking line.