To provide a mobile computing service system by which the same computing environment is realized even in a different work shop by carrying mobile media where a program is stored.

A user reserves the use of a work computer 3 in a server computer 4 from a work computer 2, receives the issued individual authenticating information and a digital signature and stores them in the mobile media 5. The user carries the mobile media 5 and fitted it to the work computer 3. The work computer 3 authenticates the user by the individual authenticating information and the digital signature stored in the mobile media 5. Then an operation is started from the mobile media 5 so as to provide the same computing environment which is the same as that in the case of using the work computer 2.