A planar antenna according to the present invention includes a plurality of antenna elements arranged thereon and characterized in that two opposite corner parts from among corners of an N-gon (N is an even number equal to or greater than four) are cut out. For example, a planar antenna has a shape in which two opposite corner parts from among four corner parts included in a rectangle (11) (N=4) are cut out. The planar antenna may be composed by combining a plurality of square antenna units on which a plurality of antenna elements are arranged.