PURPOSE: To facilitate correction of any inclination of a working electrode fitted to a holding shaft, by exciting a coil and fixing the holding shaft after clamping the electrode perpendicularly by means of a magnetic chuck.

CONSTITUTION: A holding shaft 4 for an electrode 11 is held free by a magnetic chuck while its magnetic coil 8 is not excited, and the electrode 11 fitted to the holding shaft 4 is perpendicularly clamped by pressure clamps 12, 13. The coil 8 is then excited to produce a strong magnetic field for hardening granular magnetic substance 6 filled between the holding shaft 4 and a member 5 fitted around the shaft head. This makes the holding shaft 4 fixed to a head 1, allowing to complete correction of any inclination of the electrode 11.