The present invention provides a communication device which transmits/receives data to/from a host device connected thereto via a bus having a signal line and a power line. A communication controller(18) which is always supplied with power as long as a bus (3) is connected thereto and a MCU (14) are so arranged as to use different power, and only supply of power to the MCU (14) is interrupted when a connected host device (2) comes into stand-by state. In case the connected host device (2) resumes from the stand-by state, the communication controller (18) responds to communication from the host device (2) until the MCU (14) resumes control of data transmission/reception with the host device (2).