A hearing aid apparatus is intended for sound transmission from one side of the head to the cochlea on the other side of the head for rehabilitation of patients with unilateral hearing loss. The hearing aid apparatus is based on the bone conducting principle for bone anchored hearing aids and includes a vibratory generating part that is mechanically anchored by means of osseointegration in the skull bone at the deaf side of the patient and arranged to transmit vibrations through the skull bone from the deaf side to the inner ear on the other side of the patient. The frequency characteristics of the apparatus are preferably adapted in such a way that the amplification is higher for frequencies above 1 kHz than for lower frequencies, which is in contrast to an ordinary bone anchored hearing aid.