An electronic device (100, 300) comprising: a housing (111, 311) including a first cover disposed to face a first direction, a second cover disposed to face a second direction opposite to the first direction, and a side member enclosing at least part of a space between the first and second covers; a slider (120, 320) to be inserted or withdrawn by being disposed to at least part of an area of the first cover of the housing; a flexible display (130, 330) disposed in the slider (120, 320) such that at least part thereof is exposed, and disposed to allow a display area to be expanded according to the withdrawing of the slider and allow the expanded display area (134, 834) to be hidden inside the housing; and a support body mounted in contact on a bottom face of the flexible display (130, 330), supporting the expanded display area (134, 834) as a prop when the slider is withdrawn, and disposed to be housed inside the housing (111, 311) together with the expanded display area (134, 834) when the slider is inserted; and a smoothness maintaining device; wherein the support body is fixed in contact to the expanded display area (134, 834) in an integral manner to move together with the flexible display (130, 330); wherein the smoothness maintaining device comprises: at least one first roller rotatably disposed at a first position of the housing (111, 311); a second roller facing away from the first roller, and rotatably disposed at a second position of the housing (111, 311); and at least one wire configured such that one end and the other end are movable in opposite directions that are parallel to each other, wherein the one end is coupled to one end of the support body and the other end is coupled to the other end of the support body to provide tension force for maintaining a taut state of the support body together with the first and second rollers, and configured such that the one end and the other end move in opposite directions from each other by means of the first roller to provide force for pulling each of the one end and the other end of the support body in a direction in which the both ends are away from each other. a housing (111, 311) including a first cover disposed to face a first direction, a second cover disposed to face a second direction opposite to the first direction, and a side member enclosing at least part of a space between the first and second covers; a slider (120, 320) to be inserted or withdrawn by being disposed to at least part of an area of the first cover of the housing; a flexible display (130, 330) disposed in the slider (120, 320) such that at least part thereof is exposed, and disposed to allow a display area to be expanded according to the withdrawing of the slider and allow the expanded display area (134, 834) to be hidden inside the housing; and a support body mounted in contact on a bottom face of the flexible display (130, 330), supporting the expanded display area (134, 834) as a prop when the slider is withdrawn, and disposed to be housed inside the housing (111, 311) together with the expanded display area (134, 834) when the slider is inserted; and a smoothness maintaining device; wherein the support body is fixed in contact to the expanded display area (134, 834) in an integral manner to move together with the flexible display (130, 330); wherein the smoothness maintaining device comprises: at least one first roller rotatably disposed at a first position of the housing (111, 311); a second roller facing away from the first roller, and rotatably disposed at a second position of the housing (111, 311); and at least one wire configured such that one end and the other end are movable in opposite directions that are parallel to each other, wherein the one end is coupled to one end of the support body and the other end is coupled to the other end of the support body to provide tension force for maintaining a taut state of the support body together with the first and second rollers, and configured such that the one end and the other end move in opposite directions from each other by means of the first roller to provide force for pulling each of the one end and the other end of the support body in a direction in which the both ends are away from each other. at least one first roller rotatably disposed at a first position of the housing (111, 311); a second roller facing away from the first roller, and rotatably disposed at a second position of the housing (111, 311); and at least one wire configured such that one end and the other end are movable in opposite directions that are parallel to each other, wherein the one end is coupled to one end of the support body and the other end is coupled to the other end of the support body to provide tension force for maintaining a taut state of the support body together with the first and second rollers, and configured such that the one end and the other end move in opposite directions from each other by means of the first roller to provide force for pulling each of the one end and the other end of the support body in a direction in which the both ends are away from each other.