CAP, BARREL WITH CAP, AND PREFILLED SYRINGE

PROBLEM TO BE SOLVED: To provide a cap for facilitating recognition that a nozzle part has been opened, and also to provide a barrel with a cap, and a prefilled syringe.SOLUTION: A cap body 22 is configured to be movable with the axial direction directed toward a distal end side with respect to an attachment/detachment part 23 thereby to be moved from an initial position where a seal part 21 seals a nozzle 11 to an open position being the position where the seal part 21 opens the nozzle 11. The attachment/detachment part 23 are separated from a barrel 1 when the cap body 22 is positioned at the open position. The cap body 22 and the attachment/detachment part 23 include: separation prevention parts 224, 232 to be mutually engaged thereby to prevent the cap body 22 from further moving from the open position to the distal end side and being separated from the distal end of the attachment/detachment part 23; and return prevention parts 225, 233 to be mutually engaged thereby to prevent the cap body 22 having been moved to the open position from returning to the initial position.