1. A software development production management system having a processor comprising: a management database which holds development process component data necessary to model a development process of software, and estimation parameter data used to estimate a development plan of the software; and a software development production management device which defines a software development process to be managed by consulting the development process component data held in the management database, and uses the estimation parameter data to create a software development plan from the defined software development process, wherein the software development production management device has a processing means which, upon completion of the software development, compares and evaluates an actually performed development process and the created software development plan, and makes a correction to the estimation parameter data based on results of the comparison and evaluation so that details of the actually conducted software development can be fed back to a development plan for the next time software is developed, wherein the software development production management device includes: a standard plan integration module which defines the software development process to be managed based on the development process component data, and then creates a standard software development plan containing scales and man-hours of respective work operations in the defined software development process; an execution plan creating module which assigns the work operations to workers taking into consideration the scales and man-hours of the respective work operations in the standard software development plan, and corrects details of the work operations assigned in accordance with abilities of the respective workers to create an adjusted software development plan; an actual performance information collecting module which, in developing software in accordance with the adjusted software development plan, collects actual performance information containing actual man-hour performance and actual scale performance, the actual man-hour performance being a man-hour that is actually put in by the respective workers, the actual scale performance being a scale of work that is actually done; a process completion monitoring module which, upon completion of the software development, evaluates a difference between details of the actually conducted development and the software development plan created in the execution plan creating module in a manner that includes a cause of the difference and quality of the developed software in the evaluation, and feeds results of the evaluation back to an adjustment made by the execution plan creating module; and an individual productivity evaluating module which evaluates information about productivities of the respective workers taking into consideration the actual man-hour performance and the actual scale performance, and feeds results of the evaluation back to an adjustment made by the execution plan creating module, wherein the management database holds, as a productivity empirical value, a labor cost per unit amount of a product, and records a unit labor cost of each employee, and wherein the execution plan creating module calculates productivity of each employee by dividing a product of the productivity empirical value and an actual product amount by the unit labor cost of each employee.