PURPOSE: To determine an effective oxygen concn. by using a material having a high electrical insulating characteristic to the revolving shaft of a pulling up device or part thereof and measuring the electrical conductivity of the material sticking on the insulator after growing of a crystal.

CONSTITUTION: A high frequency coil 12, refractories 5 for heat insulation and refractory powder 7 are installed in a housing 1 provided with a gas inflow port 2 and a gas outflow port 3 and a crucible 9 is installed therein. The grown crystal 11 is grown from a raw material melt 8 to a seed crystal 10 attached to the top end of the revolving shaft 4. Nitrogen contg. a small amt. of oxygen is used as the gaseous atmosphere for growing. A silver paste electrode is coated on the grown crystal 11 at about 3cm space around the darkest black part of the crystal and the electrical conductivity between such electrodes is measured, by which the effective oxygen concn. is measured.