The nature is a many-body system composed of many components, and can be considered as a composite entity showing complex phenomena arisen from various interactions among its components. Recently the efforts toward understanding complex systems, which refer to the composite entity, have appeared vigorously. The investigation on complex systems has come to be considered as an important subject of physics in the 21st century, and especially as a good example of interdisciplinary researches for which physicists, mathematicians, economists, biologists, and computer scientists can gather together. In study on complex systems, we apply the concept of a network, where complex interactions (links) between many different objects (nodes) are interwoven, to understand the structures and properties of such complex systems and collective behaviors emerging from them. Examples of such networks cover many interesting areas including technological networks such as the Internet and World Wide Web, metabolic networks representing interactions among metabolites in cells, and economic networks explaining the correlation among stock prices of companies. In our lab., we investigate the complex systems from telecommunication, economics, society, biology, etc., based on the concept of networks by exploiting the analytic and computational approach typically used in statistical physics, along with conventional phenomenological approaches.