A slot antenna apparatus including: 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 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 slot at least a part thereof to feed a radio frequency signal to the slot. The grounding conductor is formed to include at least one section at the second portion, the at least one section gradually approaches an axis passing through the slot and parallel to the radiation direction with increasing distance from the first portion.