A waveguide slot antenna is configured by a waveguide, formed by a dielectric substrate, a first conductive layer formed at a lower surface of the dielectric substrate, a second conductive layer formed at an upper surface of the dielectric substrate and provided with one or a plurality of slots, and a pair of side wall parts electrically connecting the first and second conductive layers and extending in a first direction, being provided with a power feeding part. The one or a plurality of slots include a first slot having a predetermined slot length along the first direction. The waveguide slot antenna has a structure in which, on a plan view from a second direction, the power feeding part overlaps the first slot, and the power feeding part does not deviate from a range of the slot length along the first direction.