A method comprising passing a culture solution including Bacillus subtilis or gene-recombinated Bacillus thereof through a pipe-like ceramic filter at a flow rate of 2 m/sec or less, the ceramic filter having a filter layer at an inner cylindrical surface thereof, the filter layer being made of spherical particles, and a surface roughness (Rmax) of the inner surface of the ceramic filter being 10 microns or less.
