PURPOSE: To obtain a stainproofing agent which works efficiently even when dissolved oxygen is incorporated, the purification temp. is high or the extent of staining is high and which does not generate any substance acting as a catalyst poison by using a sulfoxide compd.

CONSTITUTION: A stainproofing agent for a petroleum-base raw oil is obtd. by using a sulfoxide compd., pref. an alkyl or a phenyl sulfoxide of the formula wherein R1 and R2 are each a 1-5C alkyl or a phenyl, e.g. dimethyl sulfoxide. Since a sulfoxide is used as an effective ingredient, an excellent stainproofing effect is obtd. and it is thereby possible to prevent staining efficiently even when oxygen is incorporated, the purification temp. is high or the extent of staining is high and to obtain such an effect that no substance which acts as a catalyst poison, such as an amine, is generated by decompsn.