A slot array antenna is provided, which has a high gain and a high efficiency, is controllable of side lobes, and is adaptable to various polarized waves. The slot array antenna includes a radiation waveguide 30 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 20 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 20 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 20, 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. Slots 31L and 31R are formed in the radiation waveguide 30 such that they cut off a guide-wall current occurred due to the high-order mode, and a primary direction of the radiated electric field is oriented, for example, to the direction of propagating the electromagnetic waves in the radiation waveguide 30.