1. A load distribution control system including a plurality of computers and distributing jobs to the computers to have them execute the jobs, the load distribution system comprising: a first storage unit to be accessed by the computers to execute the jobs; a second storage unit that stores data migrated from the first storage unit after expiration of a predetermined period of time from access by at least one of the computers; a first management device that exercises management for making a computer, from among the plurality of computers, that has a lower load than the other computers, execute the jobs; a second management device that accepts the jobs requested by a user, queues the accepted jobs, and stores them sequentially in a first job queue; a third management device that sequentially fetches the jobs in the first job queue and executes, on the first storage unit and the second storage unit, preparation processing for execution of the fetched jobs by the computer having the lower load; and a second job queue that sequentially stores the jobs for which the data preparation processing has been finished, as jobs that wait to be executed by the computer having the lower load; wherein the second management device sequentially fetches the jobs from the second job queue and sends the fetched jobs to the first management device, and the first management device has the computer having the lower load execute the fetched jobs, and in the preparation processing, the third management device: if determining that I/O information corresponding to a requested job does not exist in a file in the first storage unit or in a file in the second storage unit, determines a predetermined storage capacity necessary for processing the requested job as a new job, as an expected capacity; if determining that the I/O information exists in the file of the second storage unit, determines that a capacity of the file as the expected capacity; if determining that the requested job is a retry job, determines predetermined multiples of capacity of the file corresponding to the job, as the expected capacity; judges whether or not the first storage unit has a capacity required for the requested job based on the capacity of the first storage unit that is currently being used by the computers and the expected capacity; when the judgment is negative, refers to information for a plurality of files in the first storage unit, selects at least one of plurality of files in chronological order of latest access to the files, until the amount of shortage of capacity of the first storage unit is reached, migrates data of the selected file from the first storage unit to the second storage unit, and allocates a capacity of the migrated data to the requested job; and if judging that a file storing I/O information corresponding to the requested job exists in the second storage unit, not in the first storage unit, migrates data of the file from the second storage unit to the first storage unit.