PURPOSE: To produce processed starch rich in dextrin, with a simple process in a short time at a low cost, by carrying out the gelatinization and acid- decomposition of starch utilizing the mixing, pressurizing and heating actions of an extruder.

CONSTITUTION: An aqueous solution containing starch and/or a starchy raw material (e.g. corn starch or potato starch) and 0.02W3pts.wt. (based on 100pts. wt. of the starch, etc.) of an inorganic acid (e.g. nitric acid, sulfuric acid or phosphoric acid) and/or an organic acid (e.g. citric acid or lactic acid) is supplied to an extruder, subjected to the mixing, compression and heating in the extruder to effect the gelatinization and acid-decomposition of the starch, extruded and finally dried by heating preferably at 110W200°C to a water-content of 1W8wt.% to obtain the objective processed starch. The treatment with the extruder is carried out preferably at a barrel temperature of 40W180°C, an extrusion pressure of 30W100kg/cm2 and an extruded starch temperature of 70W120°C.