Provided is a broad band antenna which is both small-sized and thin, covers a high bandwidth, such as 2.3 to 6 GHz, and has 50% band ratio or more. The broad band antenna for transmitting and receiving a broad frequency band forms a bow tie shaped slit 11 on a rectangular conductor plate 10, forms an auxiliary antenna element 14 extending along the bow tie shaped slit 11 on both sides ofa vertex angle part 12a opposing at a central part of the bow tie shaped slit 11, forms a power feeding part15 at the vertex angle part 12a on the side of the auxiliary antenna element 14, and forms a ground part 16 on the other vertex angle part 12b.