A CPU 111m segments the jobs from each of the volume rendering processing on hand, prioritize processing sequence for each job, transmits one job which has reached the processing order to the computers (21 to 2k) on the accepting side and other computers equivalent to the self computer simultaneously, and executes the job for self processing. Then, after if receiving the processing result from the computer which has completed the processing of the transmitted job the earliest, the CPU 111m issues a halt command of the job to other computers on the accepting side. At this time, if any job to be requested on hand remains uncompleted, a series of processing procedures starting with the simultaneous communication is repeated.