Radiocarbon wiggle matching method was applied for precise dating of a tephra which was designated asone of the important key beds for archaeology. The FA tephra is a pyroclastic deposit from the Shibukawaeruption of Haruna Volcano during the Kofun period of Japan. The age of FA has been investigated usingarchaeological remains and also historical records of Japan and China. It has been estimated roughly as early6th century. Three tumbled logs were found being buried in the tephra whose thickness was about 4 m. Thelogs were cut in many groups with five tree rings and every two groups were sampled for C measurement. Asthese trees were thought to have the same age from the occurrence,all data were used for wiggle matchinganalysis. The age of the outermost tree ring group is determined as AD 491-500(AD 497/+3/-6).