To provide an automatic bookmark having data communication processing function.

The present device having a Γ-shape is constituted of a transparent and elastic thin film plate or the combination of the plate and an opaque thin film plate. A base plate V is inserted into a book. A hook I is used for hanging the device from an upper part end of the back of the book. Sentences and images can be shown in a margin A of the upper part of the base plate V. A transparent page holder S holds the opened page without blocking the printed words. The outer side J and the inner side Z thereof may be inclined inward at an angle Ω or outward at an angle β respectively. The junction F of the base plate V and an arm K and the junction G of the arm K and the holder S may be of an arcuate shape, an inclined shape, or an orthogonal shape. A plurality of teeth Xn or Yn or other or a surface belt of high friction coefficient or simple adhesiveness may be applied to the single face or both faces of the base plate V in the side P. A hole Rn is used for mounting the device to a binder. A bar code, a mosaic code, an IC chip or the like enables the data communication processing.