1. A distributed processing system which has a shared storage unit shared by a plurality of computers, the system comprising: storage quota management means for managing a storage quota of each user, the storage quota limiting the total amount of data that each user is allowed to store on the shared storage unit; user identification means for identifying a particular user who has issued a write request in an attempt to store data into the shared storage unit; free quota calculation means for calculating free storage quota of the particular user identified by said user identification means; and reserve space allocation means for allocating a reserve space to a requesting computer according to the free storage quota calculated by said free quota calculation means, the reserve space being an amount of storage space on the shared storage unit which is to be managed at the discretion of the requesting computer.