An electronic device comprising: a housing comprising a first plate facing a first direction, a second plate facing a second direction opposite the first direction, and a space defined between the first plate and the second plate; a first side member surrounding a portion of the space and comprising multiple conductive portions disposed on one surface thereof and at least one first non-conductive portion disposed between the multiple conductive portions; a second side member surrounding another portion of the space at one side of the first side member and configured to be movable toward the one side of the first side member; a flexible display exposed through the first plate, an exposed width of the flexible display being adjusted based on a movement of the second side member; and a wireless communication circuit electrically connected to at least one of the multiple conductive portions and configured to transmit or receive RF signals, wherein the second side member comprises a sliding cover surface disposed to face the one surface of the first side member or located adjacent to a boundary portion of the one surface in the second direction when viewed from above the one surface, wherein the electronic device is configurable in a reduced display state and an expanded display state, wherein the reduced display state is a state in which the second side member overlaps the first side member most widely, wherein the expanded display state is a state in which the second side member overlaps the first side member most narrowly, wherein the sliding cover surface comprises at least one second non-conductive portion disposed on a portion facing the first non-conductive portion when the electronic device is in the reduced display state, and wherein the sliding cover surface does not face the first non-conductive portion when the electronic device is in the expanded display state. a housing comprising a first plate facing a first direction, a second plate facing a second direction opposite the first direction, and a space defined between the first plate and the second plate; a first side member surrounding a portion of the space and comprising multiple conductive portions disposed on one surface thereof and at least one first non-conductive portion disposed between the multiple conductive portions; a second side member surrounding another portion of the space at one side of the first side member and configured to be movable toward the one side of the first side member; a flexible display exposed through the first plate, an exposed width of the flexible display being adjusted based on a movement of the second side member; and a wireless communication circuit electrically connected to at least one of the multiple conductive portions and configured to transmit or receive RF signals, wherein the second side member comprises a sliding cover surface disposed to face the one surface of the first side member or located adjacent to a boundary portion of the one surface in the second direction when viewed from above the one surface, wherein the electronic device is configurable in a reduced display state and an expanded display state, wherein the reduced display state is a state in which the second side member overlaps the first side member most widely, wherein the expanded display state is a state in which the second side member overlaps the first side member most narrowly, wherein the sliding cover surface comprises at least one second non-conductive portion disposed on a portion facing the first non-conductive portion when the electronic device is in the reduced display state, and wherein the sliding cover surface does not face the first non-conductive portion when the electronic device is in the expanded display state.