The cationic surfactant used in the present invention is a quaternary ammonium salt of the formula (1): ##STR1## wherein R1 is an alkyl group having 12 to 22 carbon atoms, R2 and R3, which are same or different, are an alkyl group having 1 to 3 carbon atoms or a group ##STR2## in which R5 is hydrogen or methyl and n is an integer of from 1 to 3, R4 is the same as R2 or is an alkyl group having 12