Carbon and oxygen isotope abundances were determined for dolomites and calcites in Tertiary (Ogi and Otobe) and Permian (Miyama and Kuzuu) dolostones in Japan. Isotopic ratios of calcites in some of Tertiary and Permian limestones in Japan were also determined for comparison. The isotopic ratios of Tertiary dolomites are extremely low in δ¹³C values, and depend on localities. Those of Permian dolomites are distributed in narrow ranges from 2 to 6‰ in δ¹³C_PDB and from 23 to 30‰ in in δ¹⁸O_SMOW. They do not seem to depend on their localities and their mineralogical and geological features. A considerable difference in isotopic ratios between Tertiary and Permian dolomites suggests that Tertiary dolomites differ from Permian dolomites with respect to the origin. Tertiary dolomites might have been precipitated primarily from sea water in anaerobic environments, possibly owing to rise of PCO₂ by decarboxylation of organic materials. The isotopic ratios of Permian dolomites cannot be explained by the current theories on the origin of dolomite.