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 of a vial sealed with a rubber stopper, wherein a puncturing hole is formed in a center part of a cap-shaped housing that is attached to a mouth of the vial, and a rubber membrane is arranged at the puncturing hole so as to form an internal space between facing surfaces of the rubber membrane and the rubber stopper. The rubber membrane has a dome shape that is convex facing the rubber stopper.