To provide a triplate planar slot antenna for suppressing the increase of dielectric loss, the deterioration of gain and the decrease of antenna efficiency even when an antenna size is increased.

A triplate planar slot antenna 100 is formed by laying a ground plate 110, a lower layer side dielectric layer 120, a lower layer side copper-clad film substrate 130, an upper layer side dielectric layer 140, a slot plate 150 and an upper layer side copper-clad film substrate 160 sequentially from the bottom side, wherein a lower layer side copper foil piece 132 is fixed to the surface of a lower layer side insulating film 131 by a joining technology not using adhesive, and an upper layer side copper foil piece 162 is fixed to the surface of an upper layer side insulating film 161 by a joining technology not using adhesive. In a status where the copper foil pieces 132 and 162 are removed, each insulating film 131 and 161 has a dielectric constant in the range of 2.0 to 4.0, a dielectric loss tangent in the range of 0.001 to 0.01 and a thickness of 25[μm] or less.