To improve the gain of an antenna by mounting this antenna auxiliary device to a whip antenna fitted to a radio telephone set body to connect it.

An auxiliary antenna conductor 13 with λ/2 electric length is prepared so as to be fitted to the tip of a whip antenna 11 with the λ/2 electric length annexed to a portable telephone set 10. Since a pick part 12 composed of synthetic resin or the like which is used also as a protection member is formed on the tip part of a normal antenna 11, a mounting cap 14 annexed to the conductor 13 is engaged with the pick part 12. Thereby electrostatic capacity is generated between the cap 14 and the antenna 11 through the pick part 12, a high frequency current is supplied from the antenna 11 by high frequency coupling based upon the electrostatic capacity and the auxiliary antenna 13 with electric length corresponding to 1/2 high frequency current wavelength is resonated and radio waves are radiated.