An electronic device, comprising: a housing including a front plate, a rear plate, and a side member configured to surround a space between the front plate and the rear plate, wherein the front plate comprises a first flat surface portion configured to face a first direction, and wherein the rear plate comprises a second flat surface portion configured to face a second direction opposite to the first direction; a display disposed in the space between the front plate and the rear plate so as to be shown through the front plate; and a printed circuit board disposed in the space between the front plate and the rear plate and a processor on the printed circuit board, wherein the side member comprises: an external member comprising a first metallic material and comprising a first surface configured to form a surface of the side member and a second surface configured to face a direction toward the space between the front plate and the rear plate, when viewed from above the front plate; an internal member comprising a second metallic material and comprising a third surface bonded to the second surface and a fourth surface configured to face the direction toward the space between the front plate and the rear plate, wherein the internal member is surrounded by the external member when viewed from above the front plate; and a polymer member (230; 330; 430) at least partially surrounded by the internal member when viewed from above the front plate, wherein the second surface and the third surface are formed to be substantially flat surfaces, and wherein a longest distance from the first surface to the fourth surface ranges from 0.8 mm to 1.5 mm; and further comprising an interface layer (250; 350; 460; 1030) comprising the second surface and the third surface, wherein the interface layer (250; 350; 460; 1030) comprises a compound comprising the first metallic material and the second metallic material; wherein the interface layer (250; 350; 460; 1030) contains a material formed by a physical and/or chemical reaction of the first metallic material and the second metallic material at high pressure. a housing including a front plate, a rear plate, and a side member configured to surround a space between the front plate and the rear plate, wherein the front plate comprises a first flat surface portion configured to face a first direction, and wherein the rear plate comprises a second flat surface portion configured to face a second direction opposite to the first direction; a display disposed in the space between the front plate and the rear plate so as to be shown through the front plate; and a printed circuit board disposed in the space between the front plate and the rear plate and a processor on the printed circuit board, wherein the side member comprises: an external member comprising a first metallic material and comprising a first surface configured to form a surface of the side member and a second surface configured to face a direction toward the space between the front plate and the rear plate, when viewed from above the front plate; an internal member comprising a second metallic material and comprising a third surface bonded to the second surface and a fourth surface configured to face the direction toward the space between the front plate and the rear plate, wherein the internal member is surrounded by the external member when viewed from above the front plate; and a polymer member (230; 330; 430) at least partially surrounded by the internal member when viewed from above the front plate, an external member comprising a first metallic material and comprising a first surface configured to form a surface of the side member and a second surface configured to face a direction toward the space between the front plate and the rear plate, when viewed from above the front plate; an internal member comprising a second metallic material and comprising a third surface bonded to the second surface and a fourth surface configured to face the direction toward the space between the front plate and the rear plate, wherein the internal member is surrounded by the external member when viewed from above the front plate; and a polymer member (230; 330; 430) at least partially surrounded by the internal member when viewed from above the front plate, wherein the second surface and the third surface are formed to be substantially flat surfaces, and wherein a longest distance from the first surface to the fourth surface ranges from 0.8 mm to 1.5 mm; and further comprising an interface layer (250; 350; 460; 1030) comprising the second surface and the third surface, wherein the interface layer (250; 350; 460; 1030) comprises a compound comprising the first metallic material and the second metallic material; wherein the interface layer (250; 350; 460; 1030) contains a material formed by a physical and/or chemical reaction of the first metallic material and the second metallic material at high pressure.