Provided is an exposure-preventing cap of a novel and simple structure that, by being mounted on a vial, can more reliably inhibit leaking of drug solution to an outside when pulling out and collecting drug solution in a syringe from the vial. An exposure-preventing cap, which is mounted on a mouth (64) of a vial (16) sealed with a rubber stopper (66), wherein a puncturing hole (34) is formed in a center part of a cap-shaped housing (12) that is attached to a mouth (64) of the vial (16), and a rubber membrane (14) is arranged at the puncturing hole (34) so as to form an internal space (74) between facing surfaces of the rubber membrane (14) and the rubber stopper (66). The rubber membrane (14) has a dome shape that is convex facing the rubber stopper (66).
