When commonly using a receiving antenna and a transmitting antenna having mutually opposing characteristics, the present invention makes it possible to commonly use the antennas without deteriorating the characteristics of both antennas. In a communication apparatus including both of an RF-ID function circuit to perform contactless communication with an external reader/writer device and a reader/writer function circuit to perform contactless communication with an external RF-ID, there is disposed a common antenna shared therebetween. And, a transmitting and receiving resonance frequency switching circuit is connected in parallel to the common antenna; when selecting each function, a resonance frequency is automatically changed by the transmitting and receiving resonance frequency switching circuit.