Abstract: To obtain a production planning system permitting an allowance of the accuracy of production designation information between preceding and following production plans in order to follow the fluctuation of product demand forecast and received orders, a time frame comprising a series of time buckets from T1 to Tn in which the production quantity and the fluctuation width of type of product become more rough for further portion of a production plan are generated and a product quantity and type of product at a certain planned point of time are allocated to each of the time buckets from T1 to Tn. As a production period comes nearer, the time buckets are finely divided and recombined to successively decide planned production quantity and type of product. In this case, an executable production designation is automatically generated by referring to a condition storing module 3 which stores the conditions for dividing and recombining the quantity and type of product.