A plastic-made cap for a vial having a lip, the cap being useful as a separation-preventing cap for a plug that seals up an opening of the vial, includes a top wall portion having an outer circumference and centrally defining an opening, and a cylindrical skirt portion extending downwardly from the outer circumference of the top wall portion and being open at its lower end. The skirt portion has an inner diameter (a) greater than an outer diameter (b) of the lip of the vial. The skirt portion is provided on its inner wall with plural ribs formed in a direction perpendicular to the top wall portion. The plural ribs are arranged in pairs at equal angular intervals between the respective pairs of ribs. Further, engagement claws are arranged between the respective pairs of ribs such that the engagement claws can be brought into engagement with an underside of the lip of the vial.
