To decompose and convert starch rapidly by adding a specified metal (especially Mn) complex catalyst to an aqueous alkaline slurry of starch and decomposing the starch by using hydrogen peroxide.

This method comprises the steps of: feeding an aqueous starch slurry having a pH of 11-12.5, adding an effective amount of a metal complex represented by formula I (wherein L is an organic ligand having at least three N atoms coordinately bonded to a metal M; M is a transition metal of groups 6B, 7B or 8B or in the lanthanoid series in the periodic table; X is a coordinating or bridging group represented by formula II; Y is a counter ion; p is 1-4; r is 0-6; z is the charge of the complex and is an integer of 0 or greater; and s is the quotient resulting from the division of z by the number of electric charges of Y), adding an effective amount of hydrogen peroxide to the mixture, and decomposing the starch at about 0 to 55°C. M is particularly desirably manganese. It is desirable that the concentration of M is about 0.05 to 150ppm, and about 0.0075 to 15% (in terms of anhydrous hydrogen peroxide) hydrogen peroxide is added. L is exemplified by 1,4,7-timethyl-1,4,7-triazacyclononane.