Biological phenomena are generated by the cooperative and hierarchical relationships between a variety of biomolecules, such as proteins, metabolites, signaling molecules, and ions. In many cases, however, these biomolecules do not have color, and it is difficult to observe them as they are. Therefore, it is necessary to "visualize" each molecule with color or fluorescence, and to analyze the functional relationships between them. The live cell imaging technology using single fluorescent protein (FP)-based indicators has contributed to the visualization of biomolecules. Single FP-based indicators, which change their fluorescence intensity upon binding to the target molecule, have been revolutionized into multicolor indicators by a series of innovative screening methods. On the other hand, we have established an original screening method using semi-rational molecular design and molecular evolution, and have developed many single FP-based indicators for various molecules such as cAMP and glucose. In this article, we focus on single FP-based indicators and introduce their development strategy and the history of screening method.