To provide a densely wound helical antenna whose gain can be enhanced while attaining downsizing and that can properly be applied also to an antenna for transmitting/receiving a radio wave in a microwave band.

In the densely wound helical antenna comprising a straight wire shaped inner conductor 20 and a densely wound coil shaped outer conductor 30 arranged coaxially around the inner conductor 20 wherein one end of the inner conductor 20 is freely connected to a feeding side conductor of a 50Ω feeder and one end of the outer conductor 30 is freely connectable to a parasitic side conductor, the wires configuring the outer conductor 30 are wound in a form of a coil, the cross-sectional shape of which is rectangular or nearly rectangular, the longer sides configuring the coil inner diameter and the coil outer diameter, so that the height of the antenna is relatively increased and the antenna gain is improved while attaining downsizing.