The present invention addresses the problem of providing a method for producing nicotinamide mononucleotide, that produces nicotinamide mononucleotide using a single enzyme and using nucleoside monophosphate, pyrophosphate, and nicotinamide as starting materials. This problem is solved by a nicotinamide mononucleotide production method that includes at least the following steps 1) and 2): 1) a first step of producing phosphoribosyl diphosphate by the action of substantially one enzyme on nucleoside monophosphate and pyrophosphate; and 2) a second step of producing nicotinamide mononucleotide by the action of only substantially the aforementioned one enzyme on nicotinamide and the phosphoribosyl diphosphate that is the product of the first step.