To easily limit and adjust a transmission area, without revision and addition of a function to a radio transmitter itself by fitting an air-core device to a linear or a rod antenna.

The radio transmitter 1 is provided with a casing 20 in which a transmission reception printed circuit board is contained, a whip antenna 11 being a perpendicular antenna whose length is 1/4 of the wavelength stood erected from a side face of the casing 20 and an air-core device 10 mounted around a feeding part of the antenna. The air-core device 10 is composed of a dielectric magnetic body such as a ferrite core, and a fitting hole, through which the antenna is inserted is formed concentric with respect to the center. A radio wave outputted from the whip antenna 11 is attenuated by changing the number of the air-core devices and their mount positions optionally, so as to limit and adjust the transmission enable area.