For making a host computer automatically recognize a composite apparatus used with switching between functions thereof, the host 102 recognized whether a device is connected, based on a potential of cable data1. With switching between the functions of the apparatus, the device 100 temporarily stops supply of power to the cable data1 and restart the supply a predetermined time thereafter by R controller 205. When the supply of power is stopped, the host 102 assumes that the device is disconnected and deletes a driver for the device from a memory. When the supply of power is restarted thereafter, the host 102 recognizes the device 100 as a device having a new function, reads device information, and installs a driver suitable for the new device.