A slot antenna includes an insulation substrate, a metal layer provided on the insulation substrate, a slot formed in the metal layer, and a feeding part connected to the metal layer. The slot is symmetric with respect to a centerline. When an x-y coordinate system is defined on the metal layer so that the y-axis is the symmetric line, the origin is the center of the slot antenna, and the x-axis through the origin is perpendicular to the y-axis, the width of the slot in the direction of the y-axis increasing in proportion to the absolute value of the x-axis.