An automatic administration instrument includes a syringe and a partition wall in the syringe which partitions the syringe into different rooms for respectively holding plural kinds of drug solutions or a drug and a drug solution. A partition-wall driver displaces the partition wall and an injection needle is connected to the syringe. A body cap attached to the administration instrument body so as to cover the injection needle. The syringe, the partition wall, and the body cap are configured such that displacing the partition wall dissolves or mixes the drug solutions or the drug and the drug solution in a state that the injection needle is covered by the body cap.