A communication device is provided, with which packet synchronization is achieved based on input data transmitted by an external device, without using a special protocol between the communication device and the external device. An external TE attaches a CRC code to each packet before transferring data to a mobile terminal (UE). Data is bulk transferred from the external TE to the UE according to a USB protocol. At the UE, the CRC code of the input data is detected at a CRC processing unit 19. When the CRC code is detected, a first packet detecting unit 14 detects the first packet by reading a packet length. Once the first packet is detected, a second packet detection unit 15a detects succeeding packets by sequentially reading packet lengths.