The present invention provides a barrel with a cap, a pre-filled syringe, and a cap with a connector that are available for both connection-receiving bodies of the Luer lock type and connection-receiving bodies of the slip-in type. The present invention is characterized by including: a connector (3) that is detachably coupled to a cap (2) so as to be separated from the barrel (1) together with the cap (2) when the connector (3) is located at an initial position where the connector (3) is attached to a barrel (1) via the cap (2); and a connector-side engaging part 33 that engages a barrel-side engaging part (13) of the barrel (1) when the connector (3) is located at an attached position that is located closer to a proximal end in an axial direction of the barrel (1) than the initial position is, thereby allowing the connector (3) to remain attached to the barrel (1) by being detached from the cap (2) separated from a nozzle (11).