A slot antenna apparatus includes a grounding conductor having an outer edge including a first portion facing a radiation direction and a second portion other than the first portion, a one-end-open feed slot formed in the grounding conductor along the radiation direction such that an open end is provided at a center of the first portion, and a feed line including a strip conductor close to the grounding conductor and intersecting with the feed slot at at least a part thereof to feed a radio frequency signal to the feed slot. The slot antenna apparatus further comprises at least one one-end-open parasitic slot having an electrical length equivalent to one-quarter effective wavelength in a certain stop band, the parasitic slot having an open end at the second portion, and being formed in the grounding conductor so as not to intersect with the feed line.