The mushroom bed cultivation of wood-decaying edible mushrooms generates a large amount of waste mushroom beds, composed of decayed woods. The analysis of the waste mushroom beds after shiitake （Lentinus edodes） mushrooms cultivation showed that cellulose content was 25.4％, which was one of the main components. In this study, we applied TEMPO （2,2,6,6-tetramethylpiperidine-1-oxyl radical） -mediated oxidation to the waste mushroom beds of shiitake, and cellulose nanofibers were prepared. The waste mushroom beds （1.0 g） were treated with catalytic amount of TEMPO and sodium bromide under three conditions of NaClO, 20, 40 and 80 mmol. After this TEMPO-mediated oxidations, water-insoluble fractions were suspended in water, and disintegrated to individual cellulose nanofibers by sonication. When observed by the transmission electron microscopy （TEM） , these nanofibers had widths of 2-3 nm and lengths of several microns. Dry films prepared by casting of the obtained nanofiber dispersions were transparent. The highest yield of the film was obtained at the condition of 80 mmol NaClO, which was 0.18 g per 1.0 g of the waste mushroom bed （71％ toward cellulose content in the waste mushroom bed） .