It is a task of the present invention to provide an electrolytic apparatus for producing fluorine or nitrogen trifluoride by electrolyzing a hydrogen fluoride-containing molten salt, the electrolytic apparatus being advantageous in that the electrolysis can be performed without the occurrence of the anode effect even at a high current density and without the occurrence of an anodic dissolution. In the present invention, this task has been accomplished by an electrolytic apparatus for producing fluorine or nitrogen trifluoride by electrolyzing a hydrogen fluoride-containing molten salt at an applied current density of from 1 to 1,000 A/dm2, the electrolytic apparatus using a conductive diamond-coated electrode as an anode.