To provide a means for miniaturizing a storage part for a double-folded medium such as a bankbook.

In a bankbook unit 1 having the storage part 16 for storing the double-folded bankbook in an open state and in an erect positioning state, the storage part 16 includes: a front plate 31 formed with a carry-in port 38 for carrying in the bankbook; a rear plate 33 disposed oppositely to the front plate 31 with a medium storage space 35 sandwiched, having a cross-sectionally U-shaped projecting stripe 43 provided along a conveyance direction of the bankbook; and a storage medium detection sensor 41 for detecting presence of the bankbook stored in the medium storage space 35 by disposing sensor prisms 41c on the rear side of the rear plate 33 oppositely to a light emission part 41a and a light reception part 41b disposed on the rear side of the front plate 31. The projecting stripe 43 is provided in a position wherein a fold 18a of the longitudinal fold bankbook 18 of the medium storage space 35 is avoided, and the sensor prisms 41c disposed on the rear plate 33 side are stored in a recessed part 44 on the rear side of the projecting stripe 43.