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.