An electronic device comprising: a housing; a sliding plate disposed to be slidable from the housing; a display assembly including: a flexible display including a first section coupled to overlap the sliding plate, and a bendable section extending from the first section, wherein at least part of the bendable section is configured to be bent when the sliding plate is sliding, a support sheet disposed on a rear surface of the flexible display and including a second section and a third section, wherein the second section is at least partially disposed along the first section and the third section is disposed not to overlap the flexible display by being spaced apart from the second section, and at least one fastening part (710, 720, 730) disposed in the third section; at least one belt (410, 420, 430) connecting the at least one fastening part (710, 720, 730) and the sliding plate to each other; a first curved member located inside the housing to correspond to the bendable section; and a second curved member located, in contact with the at least one belt (410, 420, 430), inside the housing to correspond to the at least one belt (410, 420, 430) and spaced apart from the first curved member in a slide-out direction of the sliding plate from the housing, wherein the at least one belt (410, 420, 430) includes a plurality of fastening holes arranged in a longitudinal direction of the at least one belt (410, 420, 430), wherein at least one fastening part (710, 720, 730) includes a hook disposed on the support sheet and configured to be fastened to the plurality of holes, the plurality of fastening holes are configured to adjust tension of the at least one belt (410, 420, 430) according to a position of a fastening hole, among the plurality of fastening holes, to which the hook is fastened to. a housing; a sliding plate disposed to be slidable from the housing; a display assembly including: a flexible display including a first section coupled to overlap the sliding plate, and a bendable section extending from the first section, wherein at least part of the bendable section is configured to be bent when the sliding plate is sliding, a support sheet disposed on a rear surface of the flexible display and including a second section and a third section, wherein the second section is at least partially disposed along the first section and the third section is disposed not to overlap the flexible display by being spaced apart from the second section, and at least one fastening part (710, 720, 730) disposed in the third section; a flexible display including a first section coupled to overlap the sliding plate, and a bendable section extending from the first section, wherein at least part of the bendable section is configured to be bent when the sliding plate is sliding, a support sheet disposed on a rear surface of the flexible display and including a second section and a third section, wherein the second section is at least partially disposed along the first section and the third section is disposed not to overlap the flexible display by being spaced apart from the second section, and at least one fastening part (710, 720, 730) disposed in the third section; at least one belt (410, 420, 430) connecting the at least one fastening part (710, 720, 730) and the sliding plate to each other; a first curved member located inside the housing to correspond to the bendable section; and a second curved member located, in contact with the at least one belt (410, 420, 430), inside the housing to correspond to the at least one belt (410, 420, 430) and spaced apart from the first curved member in a slide-out direction of the sliding plate from the housing, wherein the at least one belt (410, 420, 430) includes a plurality of fastening holes arranged in a longitudinal direction of the at least one belt (410, 420, 430), wherein at least one fastening part (710, 720, 730) includes a hook disposed on the support sheet and configured to be fastened to the plurality of holes, the plurality of fastening holes are configured to adjust tension of the at least one belt (410, 420, 430) according to a position of a fastening hole, among the plurality of fastening holes, to which the hook is fastened to.