Radiocarbon 14C is produced by the galactic cosmic rays which are modulated by solar magnetic activity because most cosmic rays are charged particles. 14C is oxidized to form 14CO2 and taken by trees during the carbon cycle. So 14C content in tree-rings is retained for record of the past cosmic ray intensity and the solar activity. We have measured the 14C content in Japanese cedar tree rings from 602 to 1072 AD with 2-year resolution, and found a rapid increase in 14C content at 775 AD. This 775 AD peak is consistent with the decadal IntCal data and the 10Be record from the Antarctic ice core. Therefore, it is highly possible that this peak was recorded all over the world and it was due to the increase in the cosmic ray intensity in 775 AD. The 11-year and 14-year periodicities are detected by the Fourier analysis on the 14C content data from 602 to 1072 AD. These periodicities may come from the solar cycle (Schwabe cycle). ; 第23回名古屋大学年代測定総合研究センターシンポジウム平成22(2010)年度報告