To start an OS stored in a removable memory unit on the removable memory unit without reading the OS into a computer device.

In the removable memory unit (a USB memory unit 10), a memory part 13 comprises: a first storage area 13a recognized as a CD-ROM drive; and a second storage area 13b recognized as a flash memory. The removable memory unit has a control storage part 14 storing information for making the first storage area 13a be recognized as the CD-ROM drive, and making the second storage area 13b be recognized as the flash memory. The first storage area 13a stores: an MBR (Master Boot Record) 30; an OS starting program 31 including a search script for searching the first storage area 13a and specifying a storage place of an OS body 32; and the OS body 32 started on the USB memory unit 10 based on the OS starting program 31. The second storage area 13b stores data capable of being written or restored by the started OS body 32.