To facilitate the work of mounting a book cover and permit the mounting of the same simply without adjusting the size of the book cover even when the size or the configuration of a book is different.

A cover body 1 for covering a cover P1 and a rear cover P2 is formed. One end of the cover body 1 is bounding between the cover P1 and the first page of a main book P3 or between the last page of the main book and the rear cover P2 to fix the same. The cover body 1 is received into the inner side of the cover P1 or the rear cover P2 under folded condition while the cover P1 or the rear cover P2 is covered by the extended cover body 1 when in use. Upon book binding, the cover body 1 is bound into the inner side of eth cover P1 or the rear cover P2 together with the main book P3.