It is an object of the present invention to obtain a temperature-sensitive Bacillus subtilis natto, which has a normal spore-forming ability in a temperature zone that is generally used in a liquid culture for obtaining a Bacillus subtilis natto starter for inoculation, but has a reduced spore-forming ability in a temperature zone that is generally used in natto fermentation. It is another object of the present invention to provide natto having a small content of spores of Bacillus subtilis natto by applying existing equipment and existing steps, using the aforementioned Bacillus subtilis natto, without causing problems such as poor fermentation or microbial contamination.According to the present invention, provided is a Bacillus subtilis natto (Bacillus subtilis var. natto) mutant strain having the following properties (A) to (C):(A) when the total area of the surface of a single colony of the Bacillus subtilis natto mutant strain that has been allowed to grow by being cultured in a selective solid medium supplemented with biotin at a gas phase temperature of 49°C for 48 hours is set at 100%, with regard to the surface color of the single colony according to the Munsell Color System, the area of a region having the hue of yellow (Y), a value of 9 or more, and a chroma of 2 or less is 20% or less,(B) when the Bacillus subtilis natto mutant strain is cultured in a liquid medium for spore formation at a liquid temperature of 37°C, the number of spores of the B. subtilis natto mutant strain becomes 1 x 107 cells/ml or more, and(C) when the B. subtilis natto mutant strain is inoculated into soybeans, such that the number of spores of Bacillus subtilis natto becomes 5 x 103 cells per gram of boiled soybeans or steamed soybeans, and the temperature of the soybeans is maintained in the range of 40°C to 53°C, natto having favorable stickiness and favorable natto-specific flavor, in which the number of spores of Bacillus subtilis natto is 5 x 105 cells or less pe
