A slot array antenna is provided. In one embodiment, the slot array antenna includes a radiation waveguide having a first conductor plane in which a slot array is two-dimensionally arrayed and a second conductor plane in parallel thereto, and an introduction waveguide formed with a slot array, for introducing electromagnetic waves in a waveguide space of the radiation waveguide. Each slot of the slot array of the introduction waveguide is provided at an ½ wavelength or odd-number multiple of ½ wavelength of a wavelength inside the waveguide with respect to a direction of propagating the electromagnetic waves in the introduction waveguide, and the slots are tilted in the same direction, and thereby exciting electromagnetic waves in a high-order mode of a TE-mode in the radiation waveguide 30.