To provide a cover used for a helmet and making it possible to simply and rapidly take a cold air or rain-preventing measure for the helmet having penetrated holes and used for a bicycle, etc.

This cover 1 produced from a transparent resin can detachably be fit into the tip region 2b of a helmet 2 through an elastic deformation. Inward projections 13 capable of being fit into the prescribed penetrated holes 10 of the helmet are disposed on the inner surface of the cover 1. The inward projections 13 can energizedly be fit into the penetrated holes 10 to keep the attached state of the cover to the helmet 2.