Fallen logs of Japanese beech (Fagus crenata Blume) at various stages of decomposition were sampled from a cool temperate deciduous forest in Japan and studied for differences in the associated microfungus communities. Wood samples were directly plated onto each of two different media for the identification of fungal species. Approximately 1500 isolations were made, which represent 96 species of filamentous microfungi, consisting of 16 zygomycetes and 80 anamorphic ascomycetes. The number of species per log (α-diversity) increased with log decomposition, while dissimilarity of species composition among logs (β-diversity) showed a unimodal response with the optimum at the intermediate decay stage. The water content and nitrogen concentration of the wood were positively correlated, while the lignocellulose index and relative density were negatively correlated with α-diversity. Stepwise regression models suggested that lignocellulose index was the single most important determinant of variation in α-diversity and explained 31% of variation. Eighteen fungal species frequently isolated from the logs were classified into four groups based on their occurrence patterns. These groups occurred successively during log decomposition, and the succession in early stage of log decomposition was related with relative density, while the succession in late stage was related with water content and nitrogen concentration of wood.