A method of producing natto includes inoculating a mutant strain of Bacillus subtilis var. natto onto steamed or boiled soybeans, and fermenting the soybeans inoculated with the mutant strain by maintaining a temperature of the soybeans at 40° C. to 53° C. The number of the mutant strain spores after fermenting the soybeans is 5×105 or less spores per gram of the fermented soybeans. A single colony of the mutant strain cultured in a selective solid medium supplemented with biotin at a gas phase temperature of 49° C. for 48 hours has a region having a hue of yellow (Y), a value of 9 or more, and a chroma of 2 or less, as defined by the Munsell Color System, and a surface area of the region is 20% or less of the total surface area of the single colony.
