The method of planning a production schedule of the present invention has a production order preparing unit which, in response to an order, the completed product stock is allocated to the previous production schedule planned before an order change, and which prepares a production order. The production schedule which is previously planned is allocated to the production order which has been subjected to allocation of the completed product stock to the order, in accordance with information such as the product identification, the quantity, and the delivery time. The allocated portion of the previous production schedule is used as it is in the current production schedule.