A slot antenna apparatus includes a grounding conductor having an outer edge including a first portion and a second portion, a one-end-opened slot formed in the grounding conductor along a radiation direction such that an open end is provided at a center of the first portion, a first feed line intersecting with the slot to feed radio-frequency signals, a second feed line connected to an external circuit, and a signal processing circuit including active elements and connected between the first and second feed lines and connected to the grounding conductor. The grounding conductor is configured to be symmetric about an axis parallel to the radiation direction and passing through the slot, and is provided with a grounding terminal on the axis of symmetry at the second portion. The grounding terminal is to be connected to a ground of the external circuit.