A distributed processing system which enables a plurality of computers to make quick access to a shared storage unit. A storage quota management unit manages storage quotas, which limit the total amount of data that each user can store on the shared storage unit. When a write request to the shared storage unit is issued at a certain computer, a user identification unit identifies the requesting user. Then a free quota calculation unit calculates the remaining free storage quota of the identified user. A reserve space allocation unit allocates an appropriate reserve space to the computer according to the remaining free storage quota, allowing the computer to use the allocated reserve space at its discretion to handle the user's data write request.