After pressing a part of the exterior of a body of an administration instrument against a body region of a patient to which a drug solution is to be administered, an injection needle that is housed in the instrument body is automatically protruded from the body to insert the needle into the body region, and further, the injection needle being inserted into the body region is automatically housed in the instrument body to remove the needle from the body region. Thereby, the pain of the patient during needle insertion and needle removal is reduced, and administration at a constant speed is possible during injection of the drug solution. Furthermore, even when two kinds of drug solutions or a dissolving and mixing type drug solution are/is used, mixing can be easily and reliably carried out.