To surely prevent falling, while securing smooth working efficiency when attaching or detaching a protective cap.

This protective cap 1 is constituted of a substantially cylindrical tube part 2 for storing a cutting blade part 6 of an end mill 5 as a cutting tool and a closing part 3 for closing a distal end of the tube part 2. The tube part 2 has elliptical cross section. Projected lines 4a extending in parallel to a material axis of the tube part 2 are oppositely arranged on an internal surface of the tube part 2 on an extension line of a minor axis in the elliptical cross section. In the same way, projected lines 4b extending in parallel to the material axis of the tube part 2 are oppositely arranged on an extension line of a major axis. The projected line 4b is constituted of each one projected line, while the projected line 4a is constituted of each two projected lines, and 6 projected lines in total are formed on the internal surface of the tube part 2. The tube part 2 and the closing part 3 are integrally formed by synthetic resin such as soft vinyl chloride resin as an elastic material. The closing part 3 plays a role to prevent the end mill 5 from passing through the tube part 2 and being exposed from the distal end of the tube part 2.