The present invention relates to a sodium-sulfur electric cell structure having a specific joint between a cathode tube and an anode tube. The cathode and anode tubes are each made of an iron-nickel base alloy and are joined to each other through the medium of a ceramic ring and an aluminum alloy insert provided on both sides of the ceramic ring. A chromium or chromium-rich layer is formed at the joint boundary between the insert and each tube. The sodium-sulfur electric cell having such a structure eliminates a problem of the occurrence of cracking, improves the airtightness and enables the production of large-sized electric cells.