In a connection device (1), when a cannula holding member (7) is in a use state, a position of an end portion (31t) of an outer cylindrical portion (31) on a retracted position side does not protrude to the retracted position side, with respect to a position of an end portion (41t) of an inner cylindrical portion (41) 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.
