PURPOSE: To estimate the Al concentration in molten steel by obtaining the correlation between the Al concentration and oxygen potential in the molten steel in advance, and measuring the oxygen potential in the molten steel with an oxygen concentration battery using a solid electrolyte.

CONSTITUTION: Molten steel having the Al concentration 0-65×10-3% is prepared, and the electromotive force of the molten steel is obtained by an oxygen concentration battery using a solid electrolyte. The correlation between the electromotive force and the Al concentration in the molten steel is obtained as a regression straight line. The electromotive force is considered as the oxygen potential, the correlation of the electromotive force against the Al concentration in the molten steel is very similar to the straight line, and the Al concentration in the molten steel can be practically easily estimated from it. The difference between the Al concentration estimated from the regression straight line and the actually measured value is about the Al concentration ±6×10-3% at the maximum. The Al concentration can be accurately estimated according to this correlation.