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 1 to perform contactless communication with an external reader/writer device and a reader/writer function circuit 2 to perform contactless communication with an external RF-ID, there is disposed a common antenna 10 shared therebetween. And, a transmitting and receiving resonance frequency switching circuit 11 is connected in parallel to the common antenna 10; when selecting each function, a resonance frequency is automatically changed by the transmitting and receiving resonance frequency switching circuit 11.