To provide a plate for reinforcing a foldable cap which is capable of securing a sufficient impact resistance, penetration resistance, or the like, and being compactly stored easily.

The plate 41 for reinforcement is arranged at a front resilient plate 12 and a rear resilient plate 13 of a foldable cap body 10, where a part of the back side 12a of the front resilient plate 12 and a part of a front side 13a of the rear resilient plate 13 are coupled at a folding portion 15, and the front resilient plate 12 and the rear resilient plate 13 is flattened when they are folded, and give a cap state which is a form put on the head of a human body when they are unfolded, and is arranged at a position corresponding to the vicinity of a head top part of the foldable cap body 10 when put on the head of a human body.