The invention relates to an inserter for a medical device e.g. an infusion set or the like for intermittent or continuous administration of a therapeutical substance, such as e.g. insulin. The inserter comprises a needle hub comprising an insertion needle and two elastic elements assuring automatic insertion and automatic retraction of the insertion needle. Activation of the first elastic element (11) cause a penetrating member (6A) to be inserted sub- or transcutaneously into the skin of a patient, and the second elastic element (12) cause the penetrating member (6A) to be retracted from the skin of the patient. The first elastic element (11) is in an unloaded state before activation and upon activation the first elastic element (11) energizes the second elastic element (12).