A buried humic horizon (C-14 age between 5.4 and 6.8 kyr BP) of a volcanic ash soil in Aomori, Japan, which was collected from the depth between 147 and 187 cm at 5 cm-interval (total eight sub-horizon samples), was investigated to clarify the degree of biological transformation and mobility of soil organic carbon (SOC) fractions. The SOC fractions were prepared from each sub-horizon sample by extraction and precipitation procedures with controlling pH of the extracted solution, resulting in humin, humic acid (HA), and four fulvic acid (FA) fractions (two hydrophilic FA fractions: FA(1) and FA(2), and two hydrophobic FA fractions: FA(3) and FA(IHss)). The prepared SOC fractions were characterized by C-14 age and stable isotopic ratios of C-13 (delta C-13) and N-15 (delta N-15). The hydrophilic FA fractions showed the highest delta C-13 and delta N-15 values, indicating that these SOC fractions had been most enriched with C-13 and N-15 by biological metabolic processes. On the other hand, the HA fraction showed the lowest delta C-13 and delta N-15 values, therefore this fraction would have been less-metabolized, although HA fraction has been regarded as well-processed in general. The C-14 age of the HA fraction was almost same as the deposition age of the corresponding sub-horizon, indicating that the C in the HA fraction would have been fixed in situ right after photosynthesis by plants at the early stage of soil formation and chemically stabilized at soil surface, by fire event, etc. The average rates of vertical translocation of the SOC fractions were low (humin and HA fractions: < 1 mm per century, FA fractions: 1-4 mm per century), implying that the vertical translocation of SOC would not be the main mechanisms for forming thick humic horizons. The present study showed strong evidence of the in situ formation of SOC on soil surface, and successive up-building accumulation of soil particles containing SOC would contribute to the formation of the thick humic horizons.