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 operativity 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.
