In a connection device, when a cannula holding member is in a use state, a position of an end portion of an outer cylindrical portion on a retracted position side does not protrude to the retracted position side, with respect to a position of an end portion of an inner cylindrical portion on the retracted position side. Thereby, a connection device including a structure capable of holding a vial by holding means irrespective of the size of the vial can be provided.
