Starch cake (I) from sweet potatoes was roasted with acids, hydrolyzed with H/sub 2/SO/sub 4/, and, after neutralization, used for fermentation. Minimum viscosity was observed in mash, when 100 g of I was roasted at 150/sup 0/ for 40 minutes after spraying with 30cc. of 0.6% HCL and hydrolyzing with 500 cc. of 1% H/sub 2/SO/sub 4/ at 3 kg/sq. cm. for 40 minutes. I hydrolyzed directly with 5 times as much 1% H/sub 2/SO/sub 4/ did not become fluid nor form mash. Total sugar could be increased to 17% by the acid-roasting; 8.68% of EtOH in culture broth was obtained. Formation of fermentation-inhibiting substances or over-decomposition of starch was not observed by the treatment.