The time required to modify order allocation rules is shortened in a production planning system. A performance rule is generated, in which a manufacturing condition is set as the input and a manufacturing facility is set as the output, based on allocation performance data representing a performance of allocating the manufacturing facility for the manufacturing condition. A comparison table is generated representing a difference between the performance rule and an existing rule in which the manufacturing condition is set as the input and the manufacturing facility is set as the output. A modification rule is generated in which the existing rule is modified according to a user operation on the comparison table; and an evaluation index is calculated for production plan data corresponding to each of the existing rule and the modification rule. The rule modification unit generates the comparison table including a modification plan for the existing rule.