A slot array antenna where slots are arrayed in two directions is provided for suppressing occurrence of side lobes. An antenna includes a radiation waveguide part and stubs. The radiation waveguide part has two sheets of metal plates facing each other and outwardly radiates a radio wave from a plurality of slots formed in one of the metal plates. Each stub is arranged for every slot to extend inward of the radiation waveguide part from the face where the slots are formed, and changes a transmission mode of the radio wave within the radiation waveguide part. The slots are narrow and arranged at fixed intervals in both a longitudinal direction and a lateral direction of the slots. The stub is disposed for every slot, on either side of the slot in the lateral direction. Between the adjacent slots, the stubs are disposed on the opposite sides in the lateral direction.