Embodiments of the present invention provide a production planning method and system. Where a manufacturing apparatus is shared by a plurality of products in a production line, if a manufacturing (production) availability number is changed, a cause parameter is automatically investigated so that a production plan with high precision can be prepared in a short time. With respect to a production plan as the previous/current comparison objects, a production process used in the production plan in which a change occurs, is extracted. From the production process, a parameter item to be used in the production capacity evaluation is extracted for all products, a difference comparison is made between the value of the previous parameter and the value of the current parameter, so that the parameter item as the cause is specified. The combination of the cause parameter items is calculated, and a cause parameter list is prepared. Next, from the specified parameter list, the parameter item is sequentially changed, and the manufacturing (production) availability number is recalculated. An influence index given to the manufacturing (production) unavailability number by the cause parameter, is calculated.