PURPOSE: To prepare a manufacture plan in a short time by carrying out simulation, controlling the flow of a job in each work center according to the degree of priority in a plan preparation object period, referring to the basic information and production plan, and calculating the manufacture plan.

CONSTITUTION: When the plan preparation object period is provided, a processing time calculating means 1 calculates the processing time 8 in each process, in a nonlimited load heap up state, referring to the basic information 5 and the production plan 6, and a job progress calculating means 2 compares the in-process production information 5 and the production plan 7 in the preceding time, and the speed in the progress of each job is obtain, and the progress information 9 is calculated. A calculating means 3 calculates the priority degree 10 of each job, referring to each information, and then, a calculating means 4 develops the job described in the production plan 6 and the in process production plan 5 along the time axis, and simulation of all the processes in a production system is carried out, controlling the flow of the job in each work center, according to the priority degree 10 of the job, and a manufacture plan 11 is prepared. Thus, the manufacture plan can be prepared in a short time.