The present invention provides a highly convenient removable device and a startup method. A hub division unit 31 in a single composite device 2 allocates data exchange with the computer 1 side a plurality of devices, thereby easily realizing a plurality of functions. When connected to a USB, a recognition control unit 32 returns a signal representing a CD-ROM in a simulated manner in response to inquiry about the type of the device from the host side. When a device is mounted, a desired processing such as a program execution described in the script is automatically performed without installing a resident program for detecting a device mounting in advance in the computer side.