The invention provides a slimming skin external preparation and a cosmetic that contain a carnitine derivative having excellent skin affinity and percutaneous absorption properties and capable of enhancing the fat metabolism, and that can treat, reduce or prevent the obesity by functioning as a lipolytic agent to enhance the fat metabolism of general or local subcutaneous fat tissues. The slimming skin external preparation contains a particular carnitine derivative and preferably other slimming and skin-care ingredients.