To eliminate the need for a large storage space, by unfolding, when fitting the device on the corner of an article, one of the cut pieces of a second wall into nearly a flat plate so that a first wall forms nearly a letter L, and setting the other cut piece into an upright position to the first wall, thereby giving the shape of a solid which covers the sides and top and bottom of the corner of the article.

In use of a protecting device 1, a first cut piece 8a folded in a triangle from a fourth crease 12 is unfolded into a flat plate so that a first wall 4 folded in two forms nearly a letter L, and then a second cut piece 8b is set in an upright position to the other end 4b of the first wall 4 so that they nearly form a letter L. Consequently, the platelike member 2 takes the shape of a boxlike solid in which two sides adjacent to each other are open, and it covers and protects the sides and top and bottom of a corner of a mattress by being fitted on the corner. Further, by bending a first and second engagement pieces 15a and 15b, made in the first and second cut pieces 8a and 8b of a pair of second walls 6, outside after the platelike member 2 is unfolded into the solid shape, the first and second cut pieces 8a and 8b are kept joined.