PURPOSE: To continuously and quickly analyze the concentration of oxygen ions in a molten salt bath by measuring the absorbance of the absorption waveform resulting from the oxygen ions appearing in a region of a specific wave number in the infrared absorption spectrum of the molten salt.

CONSTITUTION: A steel band 1 to be plated is processed through a pretreatment for surface purification and activation thereof. Thereafter, the steel band 1 is sent to a plating tank 2, where the electrocrystallization by Al or Al alloy acts to bath surfaces of the steel band 1 due to the electrolysis of a molten salt plating bath 4 in the tank white the steel band goes between two confronting anode plates 3. There is provided a circulating route 8 for extracting part of the molten salt plating bath 4 from the plating tank 2 and sending back to the plating tank 2, and a pump 9 and arm infrared spectrophotometer 10 are also provided in the middle of the circulating route 8 to transfer the molten salt. The circulating route 8 is directly connected with a flow cell 11 of the spectrophotometer 10. The concentration of oxygen ions is obtained by the spectrometer 10 with the use of a calibration curve from the absorbance of the absorption waveform due to the oxygen ions in a range of 1200-1800cm-1 wave number of the infrared absorption spectrum of the molten salt.