A Portable electronic device, the portable electronic device comprising: a front cover, a display exposed through the front cover, a rear cover, a case comprising a first side portion (1031; 1131) and a second side portion (1039; 1139), wherein the first side portion further comprises a first metallic portion (231; 331) electrically connected with a communication circuit of the portable electronic device and configured to be used as an antenna, the first metallic portion including a metallic material; wherein the second side portion comprises a second metallic portion (239b; 339) spaced apart from the first metallic portion by a first gap, the second metallic portion including the metallic material; and a processor operatively coupled to the display; wherein the first side portion (1031, 1131) includes: at least one hole area (1045, 1055; 1165) defining a through-hole passing through the first side portion (1031; 1131), the through-hole having a metallic curved inner surface, a bonding layer (1041, 1051; 1161) disposed on the metallic curved inner surface, the bonding layer including an organic material and an oxidation layer of the metallic material, and an insulation member formed, by injection molding of a polymer resin, on the bonding layer such that a substantial portion of the hole area remains open, the case further comprising a first bonding insulation layer (240a; 340) disposed between the first metallic portion and the second metallic portion and including a first part, a second part, and a third part; wherein the first part of the first bonding insulation layer includes a first insulation member located in the first gap; wherein the second part of the first bonding insulation layer includes a first bonding layer in contact with the first metallic portion and the first insulation member, the first bonding layer including an organic material; and wherein the third part of the first bonding insulation layer includes a second bonding layer in contact with the first insulation member and the second metallic portion. a front cover, a display exposed through the front cover, a rear cover, a case comprising a first side portion (1031; 1131) and a second side portion (1039; 1139), wherein the first side portion further comprises a first metallic portion (231; 331) electrically connected with a communication circuit of the portable electronic device and configured to be used as an antenna, the first metallic portion including a metallic material; wherein the second side portion comprises a second metallic portion (239b; 339) spaced apart from the first metallic portion by a first gap, the second metallic portion including the metallic material; and a processor operatively coupled to the display; wherein the first side portion (1031, 1131) includes: at least one hole area (1045, 1055; 1165) defining a through-hole passing through the first side portion (1031; 1131), the through-hole having a metallic curved inner surface, a bonding layer (1041, 1051; 1161) disposed on the metallic curved inner surface, the bonding layer including an organic material and an oxidation layer of the metallic material, and an insulation member formed, by injection molding of a polymer resin, on the bonding layer such that a substantial portion of the hole area remains open, the case further comprising a first bonding insulation layer (240a; 340) disposed between the first metallic portion and the second metallic portion and including a first part, a second part, and a third part; wherein the first part of the first bonding insulation layer includes a first insulation member located in the first gap; wherein the second part of the first bonding insulation layer includes a first bonding layer in contact with the first metallic portion and the first insulation member, the first bonding layer including an organic material; and wherein the third part of the first bonding insulation layer includes a second bonding layer in contact with the first insulation member and the second metallic portion. at least one hole area (1045, 1055; 1165) defining a through-hole passing through the first side portion (1031; 1131), the through-hole having a metallic curved inner surface, a bonding layer (1041, 1051; 1161) disposed on the metallic curved inner surface, the bonding layer including an organic material and an oxidation layer of the metallic material, and an insulation member formed, by injection molding of a polymer resin, on the bonding layer such that a substantial portion of the hole area remains open, the case further comprising a first bonding insulation layer (240a; 340) disposed between the first metallic portion and the second metallic portion and including a first part, a second part, and a third part; wherein the first part of the first bonding insulation layer includes a first insulation member located in the first gap; wherein the second part of the first bonding insulation layer includes a first bonding layer in contact with the first metallic portion and the first insulation member, the first bonding layer including an organic material; and wherein the third part of the first bonding insulation layer includes a second bonding layer in contact with the first insulation member and the second metallic portion.