PURPOSE: To provide the production plan preparation assistance system which easily preparates the production plan of all processes of a multistage production line in consideration of loads on the respective processes.

CONSTITUTION: This system is equipped with a 1st production plan transmission and reception part 4 which transmits and receives production plan data to a following process, a necessary volume development part 6 which calculates the production volume of the following process according to the production plan data on the following process received by the production plan transmission and reception part 4, a production planning part 8 which plans production for obtaining the calculated volume, a process load calculation part 9 which calculates the loads on the respective processes according to the preparated production plan, and a 2nd production plan transmission and reception part 10 which transmits and receives the production plan data on the current process to and from the precedent process; when the calculated load exceeds the production ability of the current process, the production planning part 8 is prompted to correct the production plan.