A slot array antenna includes a feed waveguide plate, a radiating waveguide plate and a slot plate adhered or connected together. The feed waveguide plate is formed with a first feed port and a feed waveguide to which an electromagnetic wave is fed via the feed port. The radiating waveguide plate is formed with second feed ports communicated to the feed waveguide, and radiating waveguides communicated to the second feed ports. The slot plate is formed with slots for radiating electromagnetic waves fed via the radiating waveguides. The first feed port is positioned substantially at the center of the length of the feed waveguide. The slot array antenna allows a polarization direction of the antenna to be freely adjusted while minimizing a wasteful space otherwise allotted to peripheral circuits and is therefore miniaturized.