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 which is always supplied with power as long as a bus is connected thereto and a MCU are so arranged as to use different power, and only supply of power to the MCU is interrupted when a connected host device comes into stand-by state. In case the connected host device resumes from the stand-by state, the communication controller responds to communication from the host device until the MCU resumes control of data transmission/reception with the host device.