PURPOSE: To estimate the amt. of a fluoride component in a electrolytic bath by analyzing the electrolytic bath of the molten salt contg. a rare-earth element compd. and lithium fluoride by X-ray diffraction.

CONSTITUTION: Two graphite anodes 3 are arranged on both sides of an iron cathode 4 in an electrolytic cell 1. When a current is applied, an electrolysis reaction is caused in the molten salt 2, and the electrolyzed neodymium Nd reacts with the cathode 4 to form an Nd-Fe droplet, which is deposited as an Nd-Fe alloy 6 at the lower part of the cell 1 contg. the molten salt 2. The alloy 6 is accumulated in a metal receiving crucible 9 lined with tantalum Ta, and the alloy 6 is periodically discharged to the outside of the cell 1 as a product by an evacuating device 7. The neodymium compd. to be electrolyzed is continuously supplied to the cell 1 from a raw material feeder 8. The fluoride component in the electrolytic bath can be rapidly and accurately estimated in this way.