Casing of electronic device

A casing of an electronic device, comprising:

• a metallic housing, having an inner surface and an outer surface opposite to the inner surface, the outer surface having a back side and lateral sides connecting with the back side, the inner surface being substantially a recessed structure, and the metallic housing having a first gap and a second gap substantially located at two opposite ends of the metallic housing and being parallel with each other, wherein the first gap and the second gap each communicates the inner surface and the outer surface;
• a first non-conductive spacer, disposed in the first gap of the metallic housing;
• and
• a second non-conductive spacer, disposed in the second gap of the metallic housing,
• wherein the metallic housing further comprises at least one supporting structure on the inner surface, and the at least one supporting structure is disposed on at least one of the first gap and the second gap.