The objective of the present study was to clarify the influence of volcanic ash addition on soil carbon stocks and the carbon accumulation process in brown forest soils (BFS) in Japan. The degree of volcanic ash addition to the soil was estimated according to the acid ammonium oxalate extractable aluminum (Alox) and lithic fragment contents, and their vertical distribution patterns. The BFS was classified in order of increasing volcanic ash influence on the soil into the following types: high Alox content with no gravel (H-Alox-NGv), high Alox with a high gravel content (H-Alox-Gv), moderate Alox (M-Alox), and low Alox (L-Alox), and then analyzed for carbon content, carbon amount, carbon stock, Alox amount and pyrophosphate extractable aluminum (Alpy) amount. The correlation between the carbon and Alpy amounts and the relationship between the Alpy and Alox amounts in the BFS samples indicated that the amount of carbon is determined by Al–humus complex formation, which is defined by the active Al generated from additional volcanic ash in BFS soil samples of BFS. Therefore, soils with thicker horizons and greater amounts of Alox had higher carbon levels in deeper horizons. For this reason, soil carbon stocks at depths of 0–30 cm and 0–100 cm, and in the effective soil depth of BFS, were larger and followed the order H-Alox-NGv = H-Alox-Gv > M-Alox > L-Alox. Furthermore, successive accumulations of volcanic ash on the soil surface promoted soil carbon accumulation as a result of the development of the surface horizon in H-Alox-NGv BFS. Our results suggest that volcanic ash additions control the soil carbon accumulation of forest soil in Japan.