Abstract: In a method of manufacturing an in-the-ear hearing aid (25), a mould (23) is made of an ear canal for use as a moulding die for making a housing of the hearing aid. After the mould is made an auxiliary tool made up of a flexible tube (11) with a flange at one end is inserted into the ear canal into the proximity of the tympanic membrane of the ear. Subsequently, a curable material (19) is introduced into the ear canal. The material presses the flange against the tympanic membrane, which precludes the ingress of material between the flange and the tympanic membrane. After the material has cured the mould with the tube and the flange is removed from the ear canal. During removal of the mould the space between the tympanic membrane and the mould is in communication with the outer air via the tube, so that no partial vacuum is produced which otherwise might damage the tympanic membrane. The mould obtained is used as a moulding die for making a custom made in-the-ear hearing aid.