To provide a bookmark for surely contributing to easier taking-out of books with a simple structure.
When an auxiliary thin plate 21 is located in an in-plane posture, an entire bookmark 11 spreads within one plane. The bookmark 11 is held in a book 27. The bookmark 11 exhibits its unique function. When a thin plate body 12 is held in the book 27, and the auxiliary thin plate 21 is simultaneously positioned in an upright posture outsize a page, the auxiliary thin plate 21 protrudes from a back cover at the lower end of the back cover of the book 27. The book 27 is housed in a bookshelf. When a worker pulls the auxiliary thin plate 21 closer to him, the book 27 slides on the surface of a shelf plate 33. The book 27 is easily pulled out from the bookshelf. The book 27 is prevented from being rotated around the lower end of the back cover. The housing space of the bookshelf is suppressed low.
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