Disclosed is a wideband antenna with a lowered standing wave ratio. The wideband antenna interposes a substance whose conductivity is about 0.1 through 10.0 as an interposition between a conductive plate and a radiation conductor; and thereby, the antenna reduces reflections of signals, and achieves a wider bandwidth as well as a sufficient gain with a lowered standing wave ratio. Also, the invention realizes a thin-type wideband antenna with a wider bandwidth and a sufficient gain, by interposing a magnetic substance whose relative permeability is more than 1 through about 8 as the interposition between the conductive plate and the radiation conductor.