A medium winding device for winding a continuous medium ejected from an image forming device includes a winding member that is rotatably arranged around which the medium is wound, an actuator that rotates the winding member to wind the medium on the winding member in a rolled state, a tension generation member that applies a tension to the medium being carried to the winding member, a roll diameter obtaining unit that obtains a roll diameter of the medium in the rolled state, wherein the roll diameter changes depending on an amount of the medium wound on the winding member, and a tension adjustment unit that adjusts the tension of the medium applied by the tension generation member depending on the roll diameter obtained by the roll diameter obtaining unit.