To provide an industrial manufacturing method capable of producing a starch phosphate to get a degree of phosphatization of at least 70% by heating-roasting.

The production method of the starch phosphate ester to get a ratio of phosphatization to be at least 70% on the manufacturing of the starch phosphate is characterized by carrying out phosphatization reaction under taking out water molecule generated in a heating-roasting step out of the system, and also a method of producing a reduced dextrin phosphate and/or a reduced oligosaccharide phosphate to have a ratio of phosphatization of at least 70% characterized by carrying out phosphatization reaction of reduced dextrin and/or reduced oligosaccharide is characterized by that the phosphatization reaction is carried out under taking out water molecule generated in a heating-roasting step from a system to the out side of the system on the phosphatization reaction of reduced dextrin and/or reduced oligosaccharide.