Abstract: A priming instrument for a medical device, a medical device system, and a method for priming a medical device, which are capable of priming the medical device having a plurality of lumens that open in a living body. The medical device includes a liquid delivering lumen having a first opening portion and a discharge lumen having a second opening portion. The priming instrument includes an instrument body having a hollow portion that covers the first opening portion and the second opening portion of the medical device, the hollow portion includes a distal portion and a proximal portion, the distal portion of the hollow portion is closed, the proximal portion of the hollow portion has an insertion port into which the medical device is insertable, and the insertion port includes a sealing portion that seals an inside of the hollow portion when the medical device is inserted into the hollow portion.