About 100 samples of baked clays were collected from five kilns found in Tottori and Shimane Prefecture in San-in district and Saga Prefecture in Kyushu district, and the natural remanent magnetizations were measured. The ages of the kilns have been estimated to be between 500 A. D. and 1800 A. D. The stable natural remanent magnetization was plotted on the reference curve, which had been established by Hirooka, in order to check the secular variation of the earth's magnetic field during 500 A. D. to 1800 A. D.