To provide a paper binder, the stock of which is paper and which has a proper constitution.

In this paper binder, a pair of binding piece engaging slot holes 8 and 8, the inner end sides of longitudinal both inner and outer ends of which are piled up with through holes 7 and 7, are longitudinally disposed in an array in a second constituent piece 3. A slider 11 made of cardboard disposed on a second constituent piece in such a manner as to be supported at least on both ends of binding piece engagement slots 8, 8 along a longitudinal direction is freely movably engaged with guiding slot holes 99 made in a third constituent piece in such a state that an intermediate part allowing the slider to move and pass through is located above through holes 7, 7 and superposed on the binding piece engaging slot holes 8, 8. Further, in a fourth constituent piece 5, which superimposed on both the moving directional ends of the slider 11, second guiding slot holes 12 and 12, which correspond to the respective sliders 11 and 11, is disposed in an array so as to engage the thick parts 11A and 11A of the slider with the second guiding slot holes 12 and 12.