The present invention provides an M-1-P-producing enzyme which employs, as starting materials, an oligosaccharide or polysaccharide having a glucose polymerization degree of 5 or more and containing an .alpha.-1,4-glycosidic bond such as maltooligosaccharide, dextrin, or starch, and which enables production of a large amount of M-1-P. The M-1-P-producing enzyme can produce M-1-P from a phosphoric acid or a salt thereof and an oligosaccharide or polysaccharide having a glucose polymerization degree of 5 or more and containing an .alpha.-1,4-glycosidic bond.