An object of the present invention is to provide a method for producing nicotinamide mononucleotide (NMN) with excellent production efficiency. The method for producing NMN according to the present invention (the first aspect) comprises the step of bringing a transformant with enhanced expression of enzymes nicotinamide phosphoribosyltransferase (Nampt), phosphoribosyl pyrophosphate synthetase (Prs) and polyphosphate kinase (Ppk), a cell-free protein synthesis reaction solution having the three enzymes expressed, or a treated product thereof into contact with a mixture containing ribose-5-phosphate (R5P), nicotinamide (NAM), ATP and polyphosphate.