To provide, for a plastic cap having an engagement assurance ring with a diameter larger than that of a cap main body connected to the bottom edges of the cap main body by half-cut bridges, a plastic pilfer-proof cap whose engagement assurance ring firmly engages and half-cut bridges are easy to cut at the time of uncapping.

In the plastic pilfer-proof (fraudulent-uncapping preventable) cap in which, on the bottom edges 10a of a cap main body 10, an engagement assurance ring 30, having a diameter larger than that of the cap main body 10, which is connected to the bottom edges 10a by means of half-cut bridges 20 is provided, a series of vertical grooves (knurls) 10b and 40b are formed on the peripheral faces of the cap main body 10 and the engagement assurance ring 30.