PURPOSE: To enable the safe and economical brewing of sweet potato SHOCHU (low-class distilled spirits) all the year round.

CONSTITUTION: Sweet potato heated by steaming, boiling, etc., to gelatinize the starch is incorporated with an arbitrary acidulant selected from acidulants for food additive excluding glacial acetic acid and sodium salt-based acidulants or sweet potato is boiled in the presence of an arbitrary acidulant selected from the above acidulant for food additive to lower the average pH of the sweet potato. The sundry germ resistance is improved and the contamination with sundry germ in storage or fermentation of secondary unrefined product is prevented by the lowering of pH.