There is provided a snap-on vial cap with a protective cover, which can be removed from a cap body by a constant removal force, and can hardly be attached to the cap body again, once being removed. A cap body (5) and a protective cover (6) are molded integrally by an insert molding method or a two-color-molding method, which generates moderate adhesive strength therebetween. An annular convex portion (12, 13) having a concave section (14) formed on the outer circumferential surface thereof is provided on the cap body (5), and an engagement portion (18) for engaging with the concave section (14) is provided on the protective cover (6), thereby enabling the protective cover 6) to be easily removed from the cap body (5) with good operability while preventing the protective cover (6) from accidentally detaching. Alternatively, projections (23) provided on a top plate of the cap body (5) are engaged into a plug (4) to prevent the cap (1) from rotating.
