To produce vitamin B12 efficiently in a high content and a high recovery rate.

This method of producing vitamin B12 is provided by performing an accilimation of a middle temperature methane bacteria obtained from a digested sludge by using H2/CO2 as a medium, propagating the accilimated methane bacteria on a carrier including an inorganic nutritious salt of a small amount of a metal element by a fixed bed bioreactor, assimilating a mixed gas obtained by adding hydrogen to at least one kind of a coal gas and a biogas for reforming the mixture to methane, and at the same time, recovering cobalamine presenting in the fermented liquid as an extracellular product by using potassium cyanide as cyanocobalamine.