Tatara is a Japanese name for making furnaces in the ancient Japanese style. The natural remnant magnetism of 239 baked clay samples collected at five Tatara sites unearthed in the Shimane and Hiroshima prefectures were measured using Hirooka's secular variation curve. The dates 1670 and 1690 for the Asahi Tatara, 1670-30 AD for the first furnace of the Onji Tatara, 1780+ 25 AD for the Izumitani Tatara, 1790+20 AD for Imada Tatara, and 1350+20 AD for the Ishigami Tatara were obtained. A few indirect documentary sources seem to support the above dates. The date obtained by the thermoluminescence method for the Ishigami Tatara agreed with the above date, and the radiocarbon date of the charcoal samples obtained at the Onji Tatara also agreed with the above date. -- AATA