An antenna apparatus is provided. The antenna apparatus includes a cavity element, a radiating element, and a feeding element. The cavity element includes an opening. The radiating element is located in the opening and is disposed at a conductive layer. An outline of the radiating element and the opening form a surround slot. An imaginary rectangle has four sides respectively abutted against an external outline of the surround slot. The feeding element is disposed at another parallel conductive layer. The feeding element includes two sections. There is a coupling spacing is between a section and the radiating element to feed into the radiating element through electric field coupling. A tail end of the section is an open circuit. Another section is an initial section of the feeding element inserted into the opening. There is a shifting spacing between the another section and a central line of the imaginary rectangle.