Galactosylceramide analogues represented by the following formula (1) or (2). (wherein X and Y each represent S or O, R.sup.1 and R.sup.2 each represent an alkyl group or an alkenyl group each having 9 to 35 carbon atoms. R.sup.3 represents an alkyl group or an alkenyl group each having 2 to 30 carbon atoms.) It is provided a .beta.-glucocerebrosidase activator which is easily available, and external skin preparations and a method of activating .beta.-glucocerebrosidase, in which improvement in formation of horny layer transmission barrier is expected by activating .beta.-glucocerebrosidase so that an improved effect in rough skin is also expected.