The present invention provides a thickener which is superior in compounding stability and has high thickening action and a surfactant composition containing the thickener. The present invention relates to a thickener comprising a fatty acid amide ether compound (1) and to a surfactant composition comprising this thickener and at least one surfactant. Namely, the present invention relates to a method of increasing the viscosity of a surfactant by using this thickener. ##STR1##

wherein R¹ represents a straight-chain or branched alkyl or alkenyl group which has 5 to 21 carbon atoms and may be substituted with at least one hydroxyl group, R² represents a straight-chain or branched alkyl group having 1 to 12 carbon atoms and R³ represents an alkylene group having 2 to 12 carbon atoms.