Each print job received from a client computer 30 is divided into minimum printing units as divisional jobs, which are stored in a job folder 23b. When either of an upper spooler 23d and a lower spooler 23e has any vacancy for next allocation, one divisional job is allocated to a printer corresponding to the spooler having the vacancy. The name of the printer that has received allocation of the divisional job is written into the job folder 23b. When any allocated divisional job having an identical image file name and an identical customer ID with those of a current object divisional job is present in the job folder 23b, the current object divisional job is allocated to the printer, to which the allocated divisional job has been allocated. This arrangement allocates multiple divisional jobs for printing an identical image demanded from an identical customer to the same printer, thus ensuring substantially equal quality of resulting printed images.