A small-sized antenna device in which the conductor length can be reduced without encountering a reduction in gain. The antenna device is constructed by mounting a main antenna unit on a mounting substrate having a transmission line formed on the upper surface of the mounting substrate and also having a ground electrode formed on the back surface. One end of the transmission line is connected to a feeding terminal of the main antenna unit, and the other end of the transmission line is connected to a radio-frequency circuit of a radio communication device in which the antenna device is installed. The ground electrode is grounded.
