We analyze the interactions between human operators and control room equipment and find that several factors affect the information-gathering and decision-making processes of operators: the contents of provided information, the way information is provided, and the knowledge of operators. These factors contribute to the perceived cognitive complexity by human operators in plant operation. Based on the information theory concept, we propose an integrated framework for evaluating this complexity. The proposed framework is designed to be applied to various types of control room equipment which have different types of man-machine interface and contain different types of information. An experimental verification for the proposed framework is performed and its result shows that the framework successfully integrates various aspects of man-machine interface systems and estimates the mental workload of human operators.