A delivery shelf system includes a shelf extending from an exterior to an interior of a building, and a returnable box (RB) stored and slidably supported in the shelf. A first recessed part (FRP) is provided in an indoor-side end part of RB and a second recessed part (SRP) is provided in an outdoor-side end part thereof; a locking mechanism including a projecting part configured to be pressed and inserted into FRP or into SRP is provided at the indoor-side end part of the shelf; the projecting part is inserted into FRP while RB is stored in the shelf, so that RB is locked to the shelf; and when the projecting part is retracted from FRP by releasing the locking and RB is pulled out from the shelf into inside the building, the projecting part is inserted into SRP and the pulled-out RB is thereby stopped.