A separator of a molten salt battery made of a porous resin sheet. The separator is improved in wettability to a molten salt by giving hydrophilicity to the resin sheet. In the case of a fluororesin sheet, the sheet is impregnated with water, and irradiated with ultraviolet rays so that C—F bonds in the fluororesin are cleaved and the resultant reacts with water to generate hydrophilic groups, such as OH groups, in each surface layer thereof. The separator gains hydrophilicity through the hydrophilic groups. The separator made of the resin can be made into a bag form. In a molten salt battery having the bag-form separator, the growth of a dendrite is prevented.