According to one embodiment, an antenna apparatus includes a waveguide is formed one or more first array elements and one or more second array element, each of the one or more first array elements being formed at a position where a coupling power ratio is no less than a threshold and including a first slot and at least one reflection suppressing element which suppresses a reflection from the first slot, each of the one or more second array elements being formed at a position where the coupling power ratio is less than the threshold and including a second slot and a third slot, the coupling power ratio indicating a ratio of an electric power supplied to each of the first and second array element to an electric power of electromagnetic wave radiated from the each of the first and second array element.