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Many changes of system requirements can be continuously proposed during the life cycle of the system. Those continuous changes of system requirements can cause many kinds of difficult and costly errors throughout the whole life cycle of a system and make most of the requirements engineering methods practically useless. An effective approach to manage requirements changes is therefore necessary for successful system development in practice. The paper suggests the NRM (Need-based Requirements Management) model and a practical approach, NRM process, constructed on the NRM model to efficiently manage such continuous changes of system requirements. The NRM model has an ability to define the system requirements in 4W (Where, Who, Why, What) aspects, so that system analysts can understand the requirements accurately, although general system requirements analysis methods define them in only 2W (Who What) aspects. The NRM process, which mainly consists of V&V (verification and validation) activities, continuously watches requirement changes throughout the whole system life cycle.