PURPOSE: To provide a reinforcing piece capable of being readily mounted on a file for binding documents by the use of binding pieces, and also preventing the file and back cover from falling down and shifting.

CONSTITUTION: A reinforcing piece 101 for reinforcing a file 5 for binding documents in use of male and female binding pieces 6, 7 is provided with a back cover reinforcing plate 2 for reinforcing the back cover 52 of a file and a back cover reinforcing plate 1 for reinforcing the back cover 51 of the file formed integrally in an intersecting manner, and a back cover reinforcing plate 1 is provided with through holes 11 for passing through tongue parts 62 of the male binding pieces 6, attached to the back cover reinforcing plate 1 of the file. When the reinforcing piece 101 is secured to the file 5, the tongue parts 62 are allowed to pass through the through holes 11 and then attached to the back cover 51 by the female binding piece 7. Thus, the reinforcing piece 101 serves to reinforce the file 5 from its inside in a state bound in the file 5.